Implement ISO9660 file record timestamp decoding.

DiscImageChef.Filesystems/ISO9660/Date.cs

@@ -4,6 +4,22 @@ namespace DiscImageChef.Filesystems.ISO9660
 {
     public partial class ISO9660
     {
-        DateTime DecodeIsoDateTime(byte[] date) => throw new NotImplementedException();
+        DateTime? DecodeIsoDateTime(IsoTimestamp timestamp)
+        {
+            try
+            {
+                DateTime date = new DateTime(timestamp.Years + 1900, timestamp.Month, timestamp.Day, timestamp.Hour,
+                                             timestamp.Minute, timestamp.Second, DateTimeKind.Unspecified);
+
+                date = date.AddMinutes(timestamp.GmtOffset * 15);
+
+                return TimeZoneInfo.ConvertTimeToUtc(date, TimeZoneInfo.FindSystemTimeZoneById("GMT"));
+            }
+            catch(Exception e)
+            {
+                // ISO says timestamp can be unspecified
+                return null;
+            }
+        }
     }
 }