ANSI C.

[VLD/VMB]

difftime() accepts time_t arguments but returns a double.
Its result is expressed in SECONDS, whereas a time_t is not
necessarily expressed in seconds.  There is no portable way
to do simple arithmetic on general time_t values to produce
a time difference in seconds, hence difftime().

gmtime(), like time(), ctime(), and localtime(), takes a
pointer to a time_t because that's what they already do.
These functions have been defined (in UNIX at least) for a
long time (no pun intended).

Really, guys, the X3J11 committee has been doing a very nice
job, much better than "obviously didn't put much thought into
anything related to times".  Seems to me they put more thought
into it than some of their detractors..