Add time-zone-aware Clock helper for Meow expiry checks

Infrastructure/Clock.cs

@@ -0,0 +1,40 @@
+namespace NoticeBoard.Infrastructure;
+
+/// <summary>
+/// Single source of "now" for Meow expiry checks, so the picker (browser wall-clock,
+/// no time zone info) and the server's comparison always agree on what "now" means.
+///
+/// Configured once at startup from the "TimeZone" setting (an IANA id like
+/// "Australia/Melbourne", or "UTC"). Leaving it blank falls back to the host
+/// machine's own local time zone — the original IIS behaviour, unchanged.
+/// </summary>
+public static class Clock
+{
+    private static TimeZoneInfo _timeZone = TimeZoneInfo.Local;
+
+    public static void Configure(string? timeZoneId)
+    {
+        if (string.IsNullOrWhiteSpace(timeZoneId))
+        {
+            _timeZone = TimeZoneInfo.Local;
+            return;
+        }
+
+        try
+        {
+            _timeZone = TimeZoneInfo.FindSystemTimeZoneById(timeZoneId);
+        }
+        catch (TimeZoneNotFoundException)
+        {
+            // Unknown id (e.g. tzdata missing, or a typo) — UTC is a safe, unambiguous fallback.
+            _timeZone = TimeZoneInfo.Utc;
+        }
+        catch (InvalidTimeZoneException)
+        {
+            _timeZone = TimeZoneInfo.Utc;
+        }
+    }
+
+    /// <summary>Current time in the configured server time zone — directly comparable to a stored ExpiresAt value.</summary>
+    public static DateTime Now => TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, _timeZone);
+}