Refactor date handling in GetStationBoardAsync to prioritize user-provided date and improve timezone management

Author: abeckDev

AbeckDev.DbTimetable.Mcp/Services/TimeTableService.cs

@@ -48,9 +48,32 @@ public async Task<string> GetStationBoardAsync(
         CancellationToken cancellationToken = default)
     {
         // Format: yyMMddHHmm (e.g., 2511051830 for 2025-11-05 18:30)
-        var germanTime = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, TimeZoneInfo.FindSystemTimeZoneById("Central European Standard Time"));
-        var dateParam = date?.ToString("yyMMdd") ?? germanTime.ToString("yyMMdd");
-        var hourParam = date?.ToString("HH") ?? germanTime.ToString("HH");
+        // If date is provided by user, use it directly (user is expected to provide German time)
+        // If not provided, get current time in German timezone
+        DateTime effectiveTime;
+        if (date.HasValue)
+        {
+            effectiveTime = date.Value;
+        }
+        else
+        {
+            try
+            {
+                var tz = TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");
+                effectiveTime = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tz);
+            }
+            catch (TimeZoneNotFoundException)
+            {
+                effectiveTime = DateTime.Now; // fallback to local time if timezone not found
+            }
+            catch (InvalidTimeZoneException)
+            {
+                effectiveTime = DateTime.Now; // fallback to local time if timezone invalid
+            }
+        }
+        
+        var dateParam = effectiveTime.ToString("yyMMdd");
+        var hourParam = effectiveTime.ToString("HH");
 
         using var request = new HttpRequestMessage(HttpMethod.Get, $"plan/{evaNo}/{dateParam}/{hourParam}");
         request.Headers.Add("DB-Client-Id", _config.ClientId);