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### 8.7 `mktime`—convert time to arithmetic representation

Synopsis

    #include <time.h>
    time_t mktime(struct tm *timp);

Description
`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`.

Returns
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.

Portability
ANSI C requires `mktime`.

`mktime` requires no supporting OS subroutines.