Refactor GL period and entry date handling to use BusinessClock for accurate date resolution; add GlAccountCodeValidator for startup validation of configured account codes.

Backend/ERPCore.Tests/GlAccountCodeValidatorTests.cs

@@ -0,0 +1,158 @@
+using ERPCore.Infra.Gl;
+using ERPCore.Services.Gl;
+using ERPCore.Services.Interfaces;
+using ERPCore.System.Errors;
+using ERPCore.System.Time;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace ERPCore.Tests;
+
+/// <summary>
+/// The validator's whole value is in *which* failures it treats as fatal: a missing account code
+/// must stop the app (the config is wrong), while an unreachable GL must not (GL shares the host
+/// and may still be starting — hard-failing there turns that into a deploy crash-loop).
+/// </summary>
+public class GlAccountCodeValidatorTests
+{
+    [Fact]
+    public async Task StartAsync_Succeeds_WhenEveryConfiguredCodeExistsInGl()
+    {
+        var validator = CreateValidator(
+            new() { ["Grn:GlInventoryAccountCode"] = "1100", ["Fifo:GlClearingAccountCode"] = "2000" },
+            new StubGl(["1100", "2000", "4000"]));
+
+        await validator.StartAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task StartAsync_Throws_AndNamesEveryMissingCode()
+    {
+        var validator = CreateValidator(
+            new()
+            {
+                ["Grn:GlInventoryAccountCode"] = "1100",
+                ["SalesDayEnd:GlCashAccountCode"] = "1000-01",
+                ["Adjustment:GlLossAccountCode"] = "5100"
+            },
+            new StubGl(["1100"]));
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => validator.StartAsync(CancellationToken.None));
+
+        Assert.Contains("1000-01", ex.Message);
+        Assert.Contains("5100", ex.Message);
+        Assert.DoesNotContain("1100,", ex.Message);
+    }
+
+    /// <summary>
+    /// The deploy-killer: if GL's /accounts shape ever changes, the reader yields nothing and a naive
+    /// validator would call every configured code missing and refuse to start.
+    /// </summary>
+    [Fact]
+    public async Task StartAsync_DoesNotThrow_WhenGlReturnsNoCodesAtAll()
+    {
+        var validator = CreateValidator(
+            new() { ["Grn:GlInventoryAccountCode"] = "1100", ["Fifo:GlClearingAccountCode"] = "2000" },
+            new StubGl([]));
+
+        await validator.StartAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task StartAsync_DoesNotThrow_WhenGlIsUnreachable()
+    {
+        var validator = CreateValidator(
+            new() { ["Grn:GlInventoryAccountCode"] = "1100" },
+            new StubGl(new DomainException(ErrorCodes.GlServiceUnavailable, "unreachable", 503)));
+
+        await validator.StartAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task StartAsync_DoesNotThrow_WhenTheChartOfAccountsReadIsCancelled()
+    {
+        var validator = CreateValidator(
+            new() { ["Grn:GlInventoryAccountCode"] = "1100" },
+            new StubGl(new OperationCanceledException()));
+
+        await validator.StartAsync(CancellationToken.None);
+    }
+
+    /// <summary>Only `Gl*Code` keys are account codes — a sibling setting must not be demanded of GL.</summary>
+    [Fact]
+    public async Task StartAsync_IgnoresSettingsThatAreNotAccountCodes()
+    {
+        var gl = new StubGl(["1100"]);
+        var validator = CreateValidator(
+            new()
+            {
+                ["Grn:GlInventoryAccountCode"] = "1100",
+                ["Grn:GlPostingEnabled"] = "true",
+                ["Grn:SomeOtherCode"] = "not-an-account"
+            },
+            gl);
+
+        await validator.StartAsync(CancellationToken.None);
+
+        Assert.Equal(1, gl.Calls);
+    }
+
+    [Fact]
+    public async Task StartAsync_SkipsTheGlCall_WhenNoCodesAreConfigured()
+    {
+        var gl = new StubGl(["1100"]);
+        var validator = CreateValidator(new(), gl);
+
+        await validator.StartAsync(CancellationToken.None);
+
+        Assert.Equal(0, gl.Calls);
+    }
+
+    /// <summary>Asia/Colombo is UTC+05:30, so 20:00 UTC on the 31st is already the 1st locally.</summary>
+    [Fact]
+    public void BusinessClock_MapsAUtcInstantToTheLocalBusinessDay()
+    {
+        Assert.Equal(new DateOnly(2026, 11, 1), BusinessClock.ToBusinessDate(new DateTime(2026, 10, 31, 20, 0, 0, DateTimeKind.Utc)));
+        Assert.Equal(new DateOnly(2026, 10, 31), BusinessClock.ToBusinessDate(new DateTime(2026, 10, 31, 18, 0, 0, DateTimeKind.Utc)));
+        Assert.Equal(new DateOnly(2026, 10, 31), BusinessClock.ToBusinessDate(new DateTime(2026, 10, 31, 18, 29, 59, DateTimeKind.Utc)));
+        Assert.Equal(new DateOnly(2026, 11, 1), BusinessClock.ToBusinessDate(new DateTime(2026, 10, 31, 18, 30, 0, DateTimeKind.Utc)));
+    }
+
+    private static GlAccountCodeValidator CreateValidator(Dictionary<string, string?> settings, IGeneralLedgerService gl)
+    {
+        var configuration = new ConfigurationBuilder().AddInMemoryCollection(settings).Build();
+        var services = new ServiceCollection().AddScoped(_ => gl).BuildServiceProvider();
+        return new GlAccountCodeValidator(services, configuration, NullLogger<GlAccountCodeValidator>.Instance);
+    }
+
+    private sealed class StubGl(HashSet<string>? codes = null, Exception? throws = null) : IGeneralLedgerService
+    {
+        private readonly HashSet<string> _codes = codes ?? [];
+        private readonly Exception? _throws = throws;
+
+        public StubGl(Exception throws) : this(null, throws) { }
+
+        public int Calls { get; private set; }
+
+        public Task<HashSet<string>> ListAccountCodesAsync(CancellationToken ct)
+        {
+            Calls++;
+            return _throws is not null ? Task.FromException<HashSet<string>>(_throws) : Task.FromResult(_codes);
+        }
+
+        public Task<ERPCore.Infra.Gl.GeneralLedgerResponse> ForwardAsync(
+            HttpMethod method, string path, string? queryString, string? contentType, Stream? body, CancellationToken ct)
+            => throw new NotSupportedException();
+
+        public Task<GlJournalEntryResult> PostJournalEntryAsync(GlJournalEntryRequest request, CancellationToken ct)
+            => throw new NotSupportedException();
+
+        public Task<GlPeriod> GetPeriodByDateAsync(DateOnly date, CancellationToken ct)
+            => throw new NotSupportedException();
+
+        public Task<IReadOnlyList<GlBankAccount>> ListBankAccountsAsync(CancellationToken ct)
+            => throw new NotSupportedException();
+    }
+}

Backend/ERPCore/Infra/Gl/GlAccountCodeValidator.cs

@@ -0,0 +1,89 @@
+using ERPCore.Services.Interfaces;
+using ERPCore.System.Errors;
+
+namespace ERPCore.Infra.Gl;
+
+/// <summary>
+/// Startup guard for the GL chart of accounts. Every <c>Gl*Code</c> in configuration names an
+/// account that must already exist in GL; until now a typo or an un-seeded account only surfaced
+/// as a 404 when some user happened to trigger that one flow, mid-transaction — one flow, one
+/// discovery, repeat. This asserts the whole set once, at boot.
+/// <para>
+/// Deliberately asymmetric about failure: a <em>confirmed missing</em> code is fatal (the config is
+/// wrong and no amount of waiting fixes it), but an <em>unreachable</em> GL only logs a warning.
+/// GL runs on the same host as ERPCore, so on a host reboot it may simply not be listening yet —
+/// hard-failing there would turn a transient ordering issue into a deploy crash-loop.
+/// </para>
+/// </summary>
+public sealed class GlAccountCodeValidator(
+    IServiceProvider services,
+    IConfiguration configuration,
+    ILogger<GlAccountCodeValidator> logger) : IHostedService
+{
+    /// <summary>Configuration sections that carry GL account codes — see appsettings.json.</summary>
+    private static readonly string[] Sections =
+        ["Grn", "SalesDayEnd", "Adjustment", "SalesInvoice", "PurchaseInvoice", "Fifo"];
+
+    /// <summary>Startup must not stall on a hung GL — HttpClient's own default is 100s.</summary>
+    private static readonly TimeSpan FetchTimeout = TimeSpan.FromSeconds(10);
+
+    public async Task StartAsync(CancellationToken ct)
+    {
+        var expected = Sections
+            .SelectMany(s => configuration.GetSection(s).GetChildren())
+            .Where(c => c.Key.StartsWith("Gl", StringComparison.Ordinal) && c.Key.EndsWith("Code", StringComparison.Ordinal))
+            .Select(c => c.Value)
+            .Where(v => !string.IsNullOrWhiteSpace(v))
+            .Select(v => v!)
+            .Distinct(StringComparer.Ordinal)
+            .ToArray();
+
+        if (expected.Length == 0)
+        {
+            logger.LogWarning("No GL account codes are configured — skipping chart-of-accounts verification.");
+            return;
+        }
+
+        HashSet<string> present;
+        try
+        {
+            using var scope = services.CreateScope();
+            var gl = scope.ServiceProvider.GetRequiredService<IGeneralLedgerService>();
+
+            using var timeout = CancellationTokenSource.CreateLinkedTokenSource(ct);
+            timeout.CancelAfter(FetchTimeout);
+            present = await gl.ListAccountCodesAsync(timeout.Token);
+        }
+        catch (Exception e) when (e is DomainException or OperationCanceledException && !ct.IsCancellationRequested)
+        {
+            // Unreachable, timed out, or rejected the read — can't verify, so don't block startup.
+            // The per-flow 404 remains the backstop it has always been.
+            logger.LogWarning(e,
+                "Could not read GL's chart of accounts at startup; {Count} configured account code(s) left unverified.",
+                expected.Length);
+            return;
+        }
+
+        // A GL that answers 2xx but yields no codes at all is far more likely a response shape this
+        // reader doesn't understand than a genuinely empty chart of accounts — and treating it as
+        // "everything is missing" would take the whole app down on a contract change. Warn, don't fail.
+        if (present.Count == 0)
+        {
+            logger.LogWarning(
+                "GL returned no account codes; {Count} configured account code(s) left unverified. " +
+                "If GL's chart of accounts is not actually empty, the /accounts response shape has changed.",
+                expected.Length);
+            return;
+        }
+
+        var missing = expected.Where(c => !present.Contains(c)).ToArray();
+        if (missing.Length > 0)
+            throw new InvalidOperationException(
+                $"General Ledger is missing {missing.Length} configured account code(s): {string.Join(", ", missing)}. " +
+                "Seed them in GL's chart of accounts, or correct the Gl*Code settings in configuration.");
+
+        logger.LogInformation("GL account codes verified: {Count} present.", expected.Length);
+    }
+
+    public Task StopAsync(CancellationToken ct) => Task.CompletedTask;
+}

Backend/ERPCore/Program.cs

@@ -77,12 +77,22 @@ builder.Services.AddScoped<IAuthAltService, AuthAltService>();
 // General Ledger service proxy → external GL microservice (docs/12-GENERAL-LEDGER-INTEGRATION.md)
 builder.Services.AddHttpClient<IGeneralLedgerClient, GeneralLedgerClient>(c =>
 {
-    var baseUrl = builder.Configuration["GeneralLedgerService:BaseUrl"]
-        ?? throw new InvalidOperationException("GeneralLedgerService:BaseUrl is not configured.");
+    var baseUrl = builder.Configuration["GeneralLedgerService:BaseUrl"];
+    if (string.IsNullOrWhiteSpace(baseUrl))
+        throw new InvalidOperationException("GeneralLedgerService:BaseUrl is not configured.");
+
+    // GeneralLedgerClient builds *relative* URIs ("journal-entries", "accounts", …), so the base
+    // address must keep its full path — including /api/v1/ — and end in '/'. Without the trailing
+    // slash, Uri resolution replaces the last segment and every call lands on /api/journal-entries.
+    if (!baseUrl.EndsWith('/')) baseUrl += "/";
     c.BaseAddress = new Uri(baseUrl);
 });
 builder.Services.AddScoped<IGeneralLedgerService, GeneralLedgerService>();
 
+// Fail fast on a misconfigured chart of accounts: every Gl*Code in configuration must exist in GL,
+// otherwise the first user to touch that flow eats a 404 mid-transaction.
+builder.Services.AddHostedService<GlAccountCodeValidator>();
+
 // Current-user (audit actor). AuthHex has no sub/nameid → a claims transformation
 // JIT-provisions a local shadow user and injects the local `int` id as `nameid`.
 builder.Services.AddHttpContextAccessor();

Backend/ERPCore/Services/AdjustmentService.cs

@@ -9,6 +9,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -73,7 +74,7 @@ public sealed class AdjustmentService : IAdjustmentService
         var loss = refs.Where(r => r.Direction == Direction.Out).Sum(r => r.Value);
         if (gain <= 0 && loss <= 0) return null;
 
-        var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), ct);
+        var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), ct);
         var lines = new List<GlJournalEntryLineRequest>();
         if (gain > 0)
         {
@@ -89,7 +90,7 @@ public sealed class AdjustmentService : IAdjustmentService
         var result = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
         {
             PeriodId = period.PeriodId,
-            EntryDate = DateOnly.FromDateTime(now),
+            EntryDate = BusinessClock.ToBusinessDate(now),
             SourceModule = "ADJUSTMENT",
             Reference = docNo,
             Narration = $"Stock adjustment {docNo}",

Backend/ERPCore/Services/CountService.cs

@@ -9,6 +9,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -80,7 +81,7 @@ public sealed class CountService : ICountService
         var loss = refs.Where(r => r.Direction == Direction.Out).Sum(r => r.Value);
         if (gain <= 0 && loss <= 0) return null;
 
-        var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), ct);
+        var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), ct);
         var lines = new List<GlJournalEntryLineRequest>();
         if (gain > 0)
         {
@@ -96,7 +97,7 @@ public sealed class CountService : ICountService
         var result = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
         {
             PeriodId = period.PeriodId,
-            EntryDate = DateOnly.FromDateTime(now),
+            EntryDate = BusinessClock.ToBusinessDate(now),
             SourceModule = "ADJUSTMENT",
             Reference = docNo,
             Narration = $"Stock adjustment {docNo}",

Backend/ERPCore/Services/GeneralLedgerService.cs

@@ -49,6 +49,19 @@ public sealed class GeneralLedgerService : IGeneralLedgerService
             a.CashAccountTypeName, a.AccountNumber, a.GlAccountId, a.GlAccountCode, a.CurrencyCode)).ToList();
     }
 
+    public async Task<HashSet<string>> ListAccountCodesAsync(CancellationToken ct)
+    {
+        // No page/pageSize: GL pages this endpoint only when both are supplied, so this is the
+        // full, flat chart of accounts (child codes like "1000-01" are rows of their own —
+        // `parentAccountId` is a field, not a nested `children` array).
+        var response = await _client.SendAsync(HttpMethod.Get, "accounts", null, null, null, ct);
+        var data = ParseSuccess<GlAccountListData>(response);
+        return data.Items
+            .Select(a => a.AccountCode)
+            .Where(c => !string.IsNullOrWhiteSpace(c))
+            .ToHashSet(StringComparer.Ordinal);
+    }
+
     /// <summary>
     /// Parses GL's <c>ApiResponse</c> envelope (case-insensitively, since GL's success bodies are
     /// camelCase and its error bodies are PascalCase — docs/12 §4) and throws
@@ -96,6 +109,16 @@ public sealed class GeneralLedgerService : IGeneralLedgerService
         public int FiscalYearId { get; set; }
     }
 
+    private sealed class GlAccountListData
+    {
+        public List<GlAccountData> Items { get; set; } = [];
+    }
+
+    private sealed class GlAccountData
+    {
+        public string AccountCode { get; set; } = string.Empty;
+    }
+
     private sealed class GlBankAccountData
     {
         public string AccountType { get; set; } = string.Empty;

Backend/ERPCore/Services/GrnService.cs

@@ -9,6 +9,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -310,7 +311,7 @@ public sealed class GrnService : IGrnService
             // with it, so inventory and the ledger never diverge (user-approved "fails
             // atomically" behavior; residual risk if GL posts but the local commit still
             // fails afterward is accepted for this phase, see the integration plan).
-            var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), token);
+            var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), token);
             var glLines = new List<GlJournalEntryLineRequest>();
             decimal totalPayable = 0m;
             foreach (var line in grn.Lines.OrderBy(l => l.GrnLineId))
@@ -325,7 +326,7 @@ public sealed class GrnService : IGrnService
             var posted = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
             {
                 PeriodId = period.PeriodId,
-                EntryDate = DateOnly.FromDateTime(now),
+                EntryDate = BusinessClock.ToBusinessDate(now),
                 SourceModule = "GRN",
                 Reference = grn.DocNo,
                 Narration = $"Goods received - GRN {grn.DocNo}",

Backend/ERPCore/Services/Interfaces/IGeneralLedgerService.cs

@@ -24,4 +24,10 @@ public interface IGeneralLedgerService
 
     /// <summary>Lists GL's cash and bank accounts (default: both types).</summary>
     Task<IReadOnlyList<GlBankAccount>> ListBankAccountsAsync(CancellationToken ct);
+
+    /// <summary>
+    /// Every account code in GL's chart of accounts. Used at startup to verify the configured
+    /// <c>Gl*Code</c> settings exist — see <see cref="ERPCore.Infra.Gl.GlAccountCodeValidator"/>.
+    /// </summary>
+    Task<HashSet<string>> ListAccountCodesAsync(CancellationToken ct);
 }

Backend/ERPCore/Services/PurchaseInvoicePaymentService.cs

@@ -7,6 +7,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -61,11 +62,11 @@ public sealed class PurchaseInvoicePaymentService : IPurchaseInvoicePaymentServi
 
         var payment = await _uow.ExecuteInTransactionAsync(async token =>
         {
-            var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), token);
+            var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), token);
             var posted = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
             {
                 PeriodId = period.PeriodId,
-                EntryDate = DateOnly.FromDateTime(now),
+                EntryDate = BusinessClock.ToBusinessDate(now),
                 SourceModule = "PURCHASE_INVOICE_PAYMENT",
                 Reference = invoice.DocNo,
                 Narration = $"Payment against invoice {invoice.DocNo}",

Backend/ERPCore/Services/PurchaseInvoiceService.cs

@@ -11,6 +11,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -185,11 +186,11 @@ public sealed class PurchaseInvoiceService : IPurchaseInvoiceService
             // Reclassify the GRN clearing balance to Accounts Payable — the mirror image of
             // SalesInvoicePaymentService's AR credit — before committing, so a GL rejection
             // rolls the whole post back (same atomicity approach as GrnService.ConfirmAsync).
-            var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), token);
+            var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), token);
             var posted = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
             {
                 PeriodId = period.PeriodId,
-                EntryDate = DateOnly.FromDateTime(now),
+                EntryDate = BusinessClock.ToBusinessDate(now),
                 SourceModule = "PURCHASE_INVOICE",
                 Reference = invoice.DocNo,
                 Narration = $"Vendor invoice recognized - {invoice.DocNo}",

Backend/ERPCore/Services/PurchaseReturnService.cs

@@ -9,6 +9,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -220,11 +221,11 @@ public sealed class PurchaseReturnService : IPurchaseReturnService
             var totalValue = refs.Sum(r => r.Value);
             if (totalValue > 0)
             {
-                var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), token);
+                var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), token);
                 var glResult = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
                 {
                     PeriodId = period.PeriodId,
-                    EntryDate = DateOnly.FromDateTime(now),
+                    EntryDate = BusinessClock.ToBusinessDate(now),
                     SourceModule = "PURCHASE-RETURN",
                     Reference = docNo,
                     Narration = $"Purchase return {docNo} — stock returned to vendor",

Backend/ERPCore/Services/SalesInvoicePaymentService.cs

@@ -7,6 +7,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -220,13 +221,15 @@ public sealed class SalesInvoicePaymentService : ISalesInvoicePaymentService
 
     /// <summary>
     /// The payment's calendar day (also the GL entry date) and the timestamp stored on it. Omitted ⇒
-    /// today. One day of slack on "future": the client sends its local date, which runs ahead of UTC
-    /// for users east of it. Today keeps the actual time of entry; a back-dated payment is stamped at
-    /// that day's start.
+    /// today — the <em>business</em> day (<see cref="BusinessClock"/>), not the UTC one, or a payment
+    /// taken between midnight and 05:30 local would post to the previous day and, on the 1st, to the
+    /// previous accounting period. The day of slack on "future" is kept as a tolerance for clients
+    /// whose own clock runs ahead. Today keeps the actual time of entry; a back-dated payment is
+    /// stamped at that day's start. The returned stamp stays UTC.
     /// </summary>
     private static (DateOnly Day, DateTime Stamp) ResolvePaymentDate(DateOnly? requested, DateTime now)
     {
-        var today = DateOnly.FromDateTime(now);
+        var today = BusinessClock.ToBusinessDate(now);
         var day = requested ?? today;
         if (day > today.AddDays(1))
             throw new DomainException(ErrorCodes.Validation, "Payment date cannot be in the future.", 400);

Backend/ERPCore/Services/SalesPostingService.cs

@@ -9,6 +9,7 @@ using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.Services.Stock;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -184,7 +185,7 @@ public sealed class SalesPostingService : ISalesPostingService
         if (invoice.NetPayable <= 0 && cogs <= 0) return;
 
         var now = DateTime.UtcNow;
-        var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), ct);
+        var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), ct);
         var lines = new List<GlJournalEntryLineRequest>();
         if (invoice.NetPayable > 0)
         {
@@ -204,7 +205,7 @@ public sealed class SalesPostingService : ISalesPostingService
         var result = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
         {
             PeriodId = period.PeriodId,
-            EntryDate = DateOnly.FromDateTime(now),
+            EntryDate = BusinessClock.ToBusinessDate(now),
             SourceModule = "SALES_INVOICE",
             Reference = invoice.InvoiceNo,
             Narration = $"Sales invoice {invoice.InvoiceNo} posted",

Backend/ERPCore/Services/SalesReturnService.cs

@@ -9,6 +9,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services;
@@ -179,11 +180,11 @@ public sealed class SalesReturnService : ISalesReturnService
             var totalValue = refs.Sum(r => r.Value);
             if (totalValue > 0)
             {
-                var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(now), token);
+                var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(now), token);
                 var glResult = await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
                 {
                     PeriodId = period.PeriodId,
-                    EntryDate = DateOnly.FromDateTime(now),
+                    EntryDate = BusinessClock.ToBusinessDate(now),
                     SourceModule = "SALES-RETURN",
                     Reference = docNo,
                     Narration = $"Sales return {docNo} — stock restocked",

Backend/ERPCore/Services/Stock/FifoCostingService.cs

@@ -6,6 +6,7 @@ using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Gl;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
+using ERPCore.System.Time;
 using Microsoft.EntityFrameworkCore;
 
 namespace ERPCore.Services.Stock;
@@ -289,11 +290,11 @@ public sealed class FifoCostingService : IFifoCostingService
 
         if (entry.Value != 0)
         {
-            var period = await _gl.GetPeriodByDateAsync(DateOnly.FromDateTime(createdAt), ct);
+            var period = await _gl.GetPeriodByDateAsync(BusinessClock.ToBusinessDate(createdAt), ct);
             await _gl.PostJournalEntryAsync(new GlJournalEntryRequest
             {
                 PeriodId = period.PeriodId,
-                EntryDate = DateOnly.FromDateTime(createdAt),
+                EntryDate = BusinessClock.ToBusinessDate(createdAt),
                 SourceModule = "STOCK",
                 Reference = $"{sourceDocType}#{sourceDocId}",
                 Narration = $"{sourceDocType} {sourceDocId} — {direction} {qtyBase} @ {unitCost}",

Backend/ERPCore/System/Time/BusinessClock.cs

@@ -0,0 +1,66 @@
+namespace ERPCore.System.Time;
+
+/// <summary>
+/// The company's local wall clock (Asia/Colombo, UTC+05:30) — the single source of truth for
+/// which <em>business day</em> a posting belongs to.
+/// <para>
+/// Every GL journal entry carries an <c>EntryDate</c> and a <c>PeriodId</c> resolved from a
+/// calendar date. Deriving that date from <see cref="DateTime.UtcNow"/> is wrong east of
+/// Greenwich: between 00:00 and 05:30 local, UTC is still on the previous day, so a payment
+/// made just after midnight posts to the previous business day — and just after midnight on
+/// the 1st, to the previous <em>accounting period</em>. GL validates only that the period
+/// exists and is unlocked, not that <c>EntryDate</c> falls inside it, so that mismatch is
+/// accepted silently.
+/// </para>
+/// <para>
+/// This type is only about <em>which day a posting belongs to</em>. Persisted timestamps
+/// (<c>CreatedAt</c>, <c>PaymentDate</c>, <c>ClosedAt</c>, …) stay in UTC — MariaDB/Npgsql
+/// comparisons and every existing row depend on that.
+/// </para>
+/// </summary>
+public static class BusinessClock
+{
+    /// <summary>Sri Lanka has observed no DST since 2006, so a fixed +05:30 offset is a faithful fallback.</summary>
+    private static readonly TimeSpan ColomboOffset = TimeSpan.FromMinutes(330);
+
+    /// <summary>
+    /// Resolved once, tolerantly: slim Linux container images frequently ship without `tzdata`,
+    /// and a throwing static initialiser would surface as a <c>TypeInitializationException</c> on
+    /// the first posting rather than at boot. The custom zone keeps behaviour identical.
+    /// </summary>
+    private static readonly TimeZoneInfo Zone = ResolveZone();
+
+    private static TimeZoneInfo ResolveZone()
+    {
+        foreach (var id in new[] { "Asia/Colombo", "Sri Lanka Standard Time" })
+        {
+            try
+            {
+                return TimeZoneInfo.FindSystemTimeZoneById(id);
+            }
+            catch (Exception e) when (e is TimeZoneNotFoundException or InvalidTimeZoneException)
+            {
+                // Try the next id, then fall through to the fixed-offset zone.
+            }
+        }
+
+        return TimeZoneInfo.CreateCustomTimeZone("Asia/Colombo (fixed)", ColomboOffset, "Sri Lanka Time", "Sri Lanka Time");
+    }
+
+    /// <summary>Current local wall-clock time (<see cref="DateTimeKind.Unspecified"/> — it is not a UTC instant).</summary>
+    public static DateTime Now => TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, Zone);
+
+    /// <summary>Today's business day.</summary>
+    public static DateOnly Today => DateOnly.FromDateTime(Now);
+
+    /// <summary>
+    /// The business day a UTC instant falls on. Prefer this over <see cref="Today"/> when a
+    /// flow already captured a <c>now</c>/<c>createdAt</c>: it keeps the period lookup and the
+    /// journal's <c>EntryDate</c> derived from one instant, so the two cannot straddle midnight.
+    /// </summary>
+    public static DateOnly ToBusinessDate(DateTime utc)
+    {
+        var asUtc = utc.Kind == DateTimeKind.Utc ? utc : DateTime.SpecifyKind(utc, DateTimeKind.Utc);
+        return DateOnly.FromDateTime(TimeZoneInfo.ConvertTimeFromUtc(asUtc, Zone));
+    }
+}

Backend/ERPCore/appsettings.json

@@ -24,7 +24,7 @@
     "MaxSizeBytes": 10485760
   },
   "GeneralLedgerService": {
-    "BaseUrl": "http://localhost:6704",
+    "BaseUrl": "http://localhost:6704/api/v1/",
     "ApiKey": "1A5FE0E389C029B1FBCD2A94650CEBF0656083BF06FF48E5E11987040ABA262D"
   },
   "Grn": {