[c-prog] Double Underline Type Prefix in C?

[dplescia]

Please excuse this relatively naive question, but what is the
significance to the double-underline prefix of some types in C? For
example, looking at the time() function in MSDN, time_t is default,
but __time_32 and __time_64 can be used with the _time_32 and _time_64
functions, respectively. While I am asking questions, is there a
significance to the single-underline prefix of the _time_32 and
_time_64 function calls? Thanks!