C#/Xamarin Android - Converts a UTC DateTime to the device local Time Zone as System.DateTime. Also includes functions to convert Android native date times to System.DateTime and back again.

gistfile1.cs

		/// <summary>
		/// Converts a UTC datestamp to the local timezone
		/// </summary>
		/// <returns>The UTC to local time zone.</returns>
		/// <param name="dateTimeUtc">Date time UTC.</param>
		public DateTime ConvertUTCToLocalTimeZone(DateTime dateTimeUtc) {

			// get the UTC/GMT Time Zone
			Java.Util.TimeZone utcGmtTimeZone = Java.Util.TimeZone.GetTimeZone ("UTC");

			// get the local Time Zone
			Java.Util.TimeZone localTimeZone = Java.Util.TimeZone.Default;

			// convert the DateTime to Java type
			Date javaDate = DateTimeToNativeDate (dateTimeUtc);

			// convert to new time zone
			Date timeZoneDate = ConvertTimeZone (javaDate, utcGmtTimeZone, localTimeZone);

			// convert to systwem.datetime
			DateTime timeZoneDateTime = NativeDateToDateTime (timeZoneDate);

			return timeZoneDateTime;
		}
		
		/// <summary>
		/// Converts a System.DateTime to a Java DateTime
		/// </summary>
		/// <returns>The time to native date.</returns>
		/// <param name="date">Date.</param>
		public static Java.Util.Date DateTimeToNativeDate(DateTime date)
		{
			long dateTimeUtcAsMilliseconds = (long)date.ToUniversalTime ().Subtract (
				new DateTime (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)
			).TotalMilliseconds;
			return new Date (dateTimeUtcAsMilliseconds);
		}

		/// <summary>
		/// Converts a java datetime to system.datetime
		/// </summary>
		/// <returns>The date to date time.</returns>
		/// <param name="date">Date.</param>
		public static DateTime NativeDateToDateTime(Java.Util.Date date)
		{
			long javaDateAsMilliseconds = date.Time;
			var dateTime = new DateTime (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).Add (TimeSpan.FromMilliseconds (javaDateAsMilliseconds));
			return dateTime;
		}

		/// <summary>
		/// Converts a date between time zones
		/// </summary>
		/// <returns>The date in the converted timezone.</returns>
		/// <param name="date">Date to convert</param>
		/// <param name="fromTZ">from Time Zone</param>
		/// <param name="toTZ">To Time Zone</param>
		public static Java.Util.Date ConvertTimeZone(Java.Util.Date date, Java.Util.TimeZone fromTZ , Java.Util.TimeZone toTZ)
		{
			long fromTZDst = 0;

			if(fromTZ.InDaylightTime(date)) {
				fromTZDst = fromTZ.DSTSavings;
			}

			long fromTZOffset = fromTZ.RawOffset + fromTZDst;

			long toTZDst = 0;
			if(toTZ.InDaylightTime(date)) {
				toTZDst = toTZ.DSTSavings;
			}

			long toTZOffset = toTZ.RawOffset + toTZDst;

			return new  Java.Util.Date(date.Time + (toTZOffset - fromTZOffset));
		}

thanks a million man