What is time_t ultimately a typedef to?

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The time_t Wikipedia article article sheds some light on this. The bottom line is that the type of time_t is not guaranteed in the C specification.

The time_t datatype is a data type in
the ISO C library defined for storing
system time values. Such values are
returned from the standard time()
library function. This type is a
typedef defined in the standard
header. ISO C defines
time_t as an arithmetic type, but does
not specify any particular type,
range, resolution, or encoding for it.
Also unspecified are the meanings of
arithmetic operations applied to time
values.

Unix and POSIX-compliant systems implement the time_t type as a signed
integer (typically 32 or 64 bits wide)
which represents the number of seconds
since the start of the Unix epoch:
midnight UTC of January 1, 1970 (not
counting leap seconds). Some systems
correctly handle negative time values,
while others do not. Systems using a
32-bit time_t type are susceptible to
the Year 2038 problem.

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