Examples from HighCharts show that they expect dates to be formatted as Date.UTC(2014, 0, 1). Date.UTC actually returns milliseconds since 1 jan 1970. So you can send it as a number from your backend.
In C# you would use the following method:
/// <returns>Milliseconds since 1 jan 1970</returns>
public static long UnixTimestamp(this DateTime date)
return (long)(date – new DateTime(1970, 1, 1)).TotalMilliseconds;