fix(cs/deserialization): Align date deserialization with other implem…

…entations

This commit modifies the date deserialization process in Bebop to ensure
consistency with the other implementations and corrects a type mismatch
issue. This change complements the previous fix for date serialization,
providing a complete and correct solution for date handling across languages.

- Modify `BebopReader.ReadDate()` method to use a more precise algorithm
- Introduce constants for ticks between epochs and date mask
- Update date conversion to use milliseconds since Unix epoch
- Ensure UTC DateTime is always returned
- Fix type mismatch between ulong and long in date calculation
- Use unchecked conversion to handle potential overflow

This change resolves inconsistencies in date representation when
deserializing data.

Runtime/C#/Runtime/BebopReader.cs

@@ -14,6 +14,9 @@ namespace Bebop.Runtime
     /// </summary>
     public ref struct BebopReader
     {
+        private const long TicksBetweenEpochs = 621355968000000000L;
+        private const ulong DateMask = 0x3fffffffffffffffUL;
+
         // ReSharper disable once InconsistentNaming
         private static readonly UTF8Encoding UTF8 = new();
 
@@ -193,9 +196,13 @@ public double ReadFloat64()
         /// </summary>
         /// <returns>A UTC <see cref="DateTime"/> instance.</returns>
         [MethodImpl(BebopConstants.HotPath)]
-        public DateTime ReadDate() =>
-            // make sure it always reads it as UTC by setting the first bits to `01`.
-            DateTime.FromBinary((ReadInt64() & 0x7fffffffffffffff) | 0x4000000000000000);
+        public DateTime ReadDate()
+        {
+            ulong rawTicks = ReadUInt64() & DateMask;
+            long ticks = unchecked((long)rawTicks);
+            long ms = (ticks - TicksBetweenEpochs) / 10000L;
+            return new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddMilliseconds(ms);
+        }
 
         /// <summary>
         ///     Reads a length prefixed string from the underlying buffer and advances the current position by that many bytes.