The ESRI link says that it is returning milliseconds. It looks like the date you posted is correct if they returned seconds.
I am wondering if you left off a digit in the number you posted?
Here is the code to perform the conversion:
static void Main(string[] args)
{
const long millisecondsSinceEpoch = 13941214241;
var time = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
var expires = time.AddMilliseconds(millisecondsSinceEpoch);
Console.WriteLine(expires);
}.