Add browser IANA-to-Windows time zone conversion tests and docs

Co-authored-by: kendaleiv <1012917+kendaleiv@users.noreply.github.com>

Author: Copilot

AtTimeZone.Tests/AtTimeZoneTests.cs

@@ -228,6 +228,61 @@ public async Task ConvertUtcToIndiaStandardTime_IanaTimeZone()
         Assert.Equal(new DateTimeOffset(2024, 1, 15, 17, 30, 0, TimeSpan.FromHours(5.5)), result);
     }
 
+    [SkippableFact]
+    public async Task BrowserIanaTimeZone_ConvertedToWindowsTimeZone_ProducesSameResult()
+    {
+        SkipIfIanaNotSupported();
+
+        // Browsers provide IANA time zones via the Intl API:
+        // const timeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+        // For example, a user in Chicago would send "America/Chicago".
+        var ianaTimeZone = "America/Chicago";
+
+        Assert.True(
+            TimeZoneInfo.TryConvertIanaIdToWindowsId(ianaTimeZone, out var windowsTimeZone),
+            $"Failed to convert IANA time zone '{ianaTimeZone}' to a Windows time zone.");
+
+        var sql = $"""
+            SELECT
+                CAST('2024-01-15 12:00:00 +00:00' AS datetimeoffset) AT TIME ZONE '{windowsTimeZone}' AS Windows,
+                CAST('2024-01-15 12:00:00 +00:00' AS datetimeoffset) AT TIME ZONE '{ianaTimeZone}' AS Iana
+            """;
+
+        await using var connection = new SqlConnection(db.ConnectionString);
+        await connection.OpenAsync();
+        await using var command = new SqlCommand(sql, connection);
+        await using var reader = await command.ExecuteReaderAsync();
+        await reader.ReadAsync();
+
+        var windowsResult = reader.GetDateTimeOffset(0);
+        var ianaResult = reader.GetDateTimeOffset(1);
+
+        Assert.Equal(windowsResult, ianaResult);
+    }
+
+    [Theory]
+    [InlineData("America/New_York", "Eastern Standard Time")]
+    [InlineData("America/Chicago", "Central Standard Time")]
+    [InlineData("America/Denver", "Mountain Standard Time")]
+    [InlineData("America/Los_Angeles", "Pacific Standard Time")]
+    [InlineData("Europe/London", "GMT Standard Time")]
+    [InlineData("Europe/Berlin", "W. Europe Standard Time")]
+    [InlineData("Asia/Tokyo", "Tokyo Standard Time")]
+    [InlineData("Australia/Sydney", "AUS Eastern Standard Time")]
+    public void IanaToWindowsTimeZoneConversion_ReturnsExpectedWindowsId(
+        string ianaId, string expectedWindowsId)
+    {
+        // Browsers send IANA time zones via:
+        // const timeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+        //
+        // .NET can convert these to Windows time zone IDs for SQL Server AT TIME ZONE:
+        Assert.True(
+            TimeZoneInfo.TryConvertIanaIdToWindowsId(ianaId, out var windowsId),
+            $"Failed to convert IANA time zone '{ianaId}' to a Windows time zone.");
+
+        Assert.Equal(expectedWindowsId, windowsId);
+    }
+
     [Fact]
     public async Task InvalidTimeZoneName_ThrowsSqlException()
     {

README.md

@@ -30,9 +30,31 @@ A SQL Server 2022 container is started automatically via Testcontainers.
 | Windows and IANA produce same result | `Eastern Standard Time` | `America/New_York` |
 | Invalid time zone name — throws error | _(invalid name)_ | |
 | IANA time zone when unsupported — throws error | | `America/New_York` |
+| Browser IANA to Windows conversion — same result | _(converted from IANA)_ | `America/Chicago` |
+| IANA to Windows ID mapping (C# only, no SQL) | Multiple | Multiple |
 
 IANA time zone tests are skipped automatically when the SQL Server instance does not support IANA time zone names.
 
 ## PostgreSQL Comparison
 
 PostgreSQL supports **only IANA time zone names** (e.g. `America/New_York`, `Europe/Berlin`) in its `AT TIME ZONE` clause — Windows time zone names like `Eastern Standard Time` are **not recognized** and will produce an error. You can query `pg_timezone_names` to see supported names.
+
+## Browser Time Zone to SQL Server
+
+Browsers expose the user's IANA time zone via the [Intl API](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/resolvedOptions):
+
+```js
+const timeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+// e.g. "America/Chicago"
+```
+
+In C#, convert the IANA time zone to a Windows time zone ID for use with SQL Server `AT TIME ZONE`:
+
+```csharp
+if (TimeZoneInfo.TryConvertIanaIdToWindowsId(ianaTimeZone, out var windowsTimeZone))
+{
+    // Use windowsTimeZone with AT TIME ZONE in SQL Server
+}
+```
+
+This allows applications to accept IANA time zones from the browser and convert them for SQL Server instances that only support Windows time zone names. SQL Server 2022+ may also accept IANA names directly.