I just found this in the ACE_Time_Value.h header.

// Static constant to remove time skew between FILETIME and POSIX
// time.  POSIX and Win32 use different epochs (Jan. 1, 1970 v.s.
// Jan. 1, 1601).  The following constant defines the difference
// in 100ns ticks.
// In the beginning (Jan. 1, 1601), there was no time and no
// computer.
// And Bill said: "Let there be time," and there was time....

I wonder why Microsoft chose that epoch. Or why Posix chose theirs. 1970 seems way too recent to be a good epoch.