This is how WCF essentially serializes DateTime values (note that non-UTC values include information about current time zone)
public static string MsJson(DateTime value)
{
    long unixEpochTicks = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).Ticks;
    long ticks = (value.ToUniversalTime().Ticks - unixEpochTicks) / 10000;
    if (value.Kind == DateTimeKind.Utc)
    {
        return String.Format("/Date({0})/", ticks);
    }
    else
    {
        TimeSpan ts = TimeZone.CurrentTimeZone.GetUtcOffset(value.ToLocalTime());
        string sign = ts.Ticks < 0 ? "-" : "+";
        int hours = Math.Abs(ts.Hours);
        string hs = (hours < 10) 
            ? "0" + hours 
            : hours.ToString(CultureInfo.InvariantCulture);
        int minutes = Math.Abs(ts.Minutes);
        string ms = (minutes < 10) 
            ? "0" + minutes 
            : minutes.ToString(CultureInfo.InvariantCulture);
        return string.Format("/Date({0}{1}{2}{3})/", ticks, sign, hs, ms);
    }
}