C Language and Libraries Help (clang.hlp) (Table of Contents; Topic list)
mktime
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─────Run-Time Library───────────────────────────────────────────────────────
 
     The mktime function converts the time structure pointed to by
     <timeptr> into a fully defined structure with "normalized" values.
     Then the function converts the contents of the structure into a
     time_t calendar time value. The structure at <timeptr> may be
     incomplete. The mktime function handles dates in any time zone
     from midnight, December 31, 1899 to midnight, February 5, 2036.
 
     The converted time has the same encoding as the values returned by
     the time function. The original values of the tm_wday and tm_yday
     components of the <timeptr> structure are ignored, and the
     original values of the other components are not restricted to
     their normal ranges.
 
     If successful, mktime sets the values of tm_wday and tm_yday
     appropriately. It sets the other components to represent the
     specified calendar time, but their values are forced to the
     normal ranges. The final value of tm_mday is not set until tm_mon
     and tm_year are determined.
 
     Note that the gmtime and localtime functions use a single
     statically allocated buffer for the conversion. If you supply this
     buffer to mktime, the previous contents will be destroyed.
 
     Return Value
 
     The mktime function returns the specified calendar time encoded as
     a value of type time_t. If <timeptr> references a date before
     midnight, December 31, 1899, mktime returns -1 cast as type time_t.
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