Fix timestamp cast to datetime (#226)

* Change TimeStamp to a readonly struct record in order to avoid unnecessary allocations.

* Change TimeStamp to a readonly struct record in order to avoid unnecessary allocations.

* reverting to origin

* Convert TimeStamp to DateTime using Unix milliseconds instead of ticks

Author: AmirShitrit

src/NRedisStack/TimeSeries/DataTypes/TimeStamp.cs

@@ -76,7 +76,7 @@ public static implicit operator long(TimeStamp ts) =>
         /// Implicit cast from TimeStamp to DateTime.
         /// </summary>
         /// <param name="timeStamp">TimeStamp</param>
-        public static implicit operator DateTime(TimeStamp timeStamp) => new DateTime(timeStamp);
+        public static implicit operator DateTime(TimeStamp timeStamp) => DateTimeOffset.FromUnixTimeMilliseconds(timeStamp).DateTime;
 
         /// <summary>
         /// Equality of TimeSeriesTuple objects