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`mktime'--convert time to arithmetic representation

     #include <time.h>
     time_t mktime(struct tm *TIMP);
`mktime' assumes the time at TIMP is a local time, and converts its
representation from the traditional representation defined by `struct
tm' into a representation suitable for arithmetic.
   `localtime' is the inverse of `mktime'.
If the contents of the structure at TIMP do not form a valid calendar
time representation, the result is `-1'.  Otherwise, the result is the
time, converted to a `time_t' value.
ANSI C requires `mktime'.
   `mktime' requires no supporting OS subroutines.