JavaScript Date constructor accepts number of milliseconds since Unix epoch (1 January 1970 00:00:00 UTC). Here’s C# extension method that converts .Net DateTime object to JavaScript date:

public static class DateTimeJavaScript
{
   private static readonly long DatetimeMinTimeTicks =
      (new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).Ticks;

   public static long ToJavaScriptMilliseconds(this DateTime dt)
   {
      return (long)((dt.ToUniversalTime().Ticks - DatetimeMinTimeTicks) / 10000);
   }
}

Usage:

var dt = new Date(<%= DateTime.Today.ToJavaScriptMilliseconds() %>);
alert(dt);

Reference

Java System.currentTimeMillis() equivalent in C#