You do not say what language are you using, if it is a .NET language, you can use: http://msdn.microsoft.com/en-us/library/z2xf7zzk.aspx for that constructor the input is in nanoseconds (are you sure that your number is in milliseconds and not in nanoseconds?).
If you are sure it is in milliseconds, the conversion to nanoseconds should be easy: 1 millisecond = 1 000 000 nanoseconds.
But I have the feeling that those are nanoseconds and not milliseconds...
Now that you have told us that it is in microseconds:
C# Example from decimal to yyyy dd MM hh:mm:ss
long microseconds = 63370738175000000;
long ticks = microseconds * 10;
DateTime timestamp = new DateTime(ticks);
Console.WriteLine(timestamp.ToString("yyyy dd MM hh:mm:ss"));
It prints:
2009 20 02 02:49:35
The other way around from yyyy dd MM hh:mm:ss to decimal
String dateString = "2009 20 02 02:49:35";
DateTime timestamp = DateTime.ParseExact(dateString, "yyyy dd MM hh:mm:ss",CultureInfo.CurrentCulture);
long ticks = timestamp.Ticks;
long microseconds = ticks / 10;
Console.WriteLine(microseconds);
It prints:
63370694975000000
And if you want it in hexadecimal just write:
Console.WriteLine(microseconds.ToString("X"));
Then it will print:
E1234FB3278DC0
If you want the answer in another programming language, please add that to you question.