From https://stackoverflow.com/questions/18614810/how-to-convert-active-directory-pwdlastset-to-date-time
According to the MSDN documentation:
This value is stored as a large integer that represents the number of 100 nanosecond intervals since January 1, 1601 (UTC).
This aligns perfectly with
DateTime.FromFileTimeUtc, as described here.
And I'm not sure why you feel the need to do the low level manipulation of the integer. I think you could just cast it.
So just do:
long value = (long)objResult.Properties["pwdLastSet"][0];
DateTime pwdLastSet = DateTime.FromFileTimeUtc(value);