For ISO standard C++...
<quote>
The C++ standard does not say anything about this, other than to adopt
the C standard function by reference.  Here is what the C standard
says, from the 1999 C update:

      7.23.2.4  The time function

      Synopsis

      [#1]

              #include <time.h>
              time_t time(time_t *timer);

      Description

      [#2] The time function determines the current calendar time.
      The encoding of the value is unspecified.

And here is the definition of the time_t type:

      [#3] The types declared are size_t (described in 7.17);

              clock_t

      and

              time_t

      which  are  arithmetic  types capable of representing times;

Standard C, and therefore standard C++, do not specify what arithmetic
type (could be signed or unsigned long, (or long long in C), or float
or double or long double.  No definition is imposed as to what the
resolution or base time is at all.

It is not portable to make any assumptions at all about a time_t, or
to do anything with it other than pass it to functions defined to
accept it, like ctime().

The POSIX standard, observed on most UNIX systems, specifies that
time() returns the number of seconds since 1-Jan-1970 in an
arithmetic, but again does not specify which arithmetic type or
whether it is integral or floating point.  Note that POSIX is an
extension to the C and C++ language standards.

An implementation that expresses time in seconds elapsed since
1-Jan-1900 is perfectly acceptable to ANSI/ISO C and C++, if not to
POSIX.
</quote>

If you are using C++/CLI with .NET, search the MSDN library for "date and
time".

http://msdn2.microsoft.com/en-us/netframework/default.aspx