The mktime function converts a local-time structure (struct tm)
    pointed to by timeptr, to a time in seconds since the Epoch (a
    time_t variable), in the same manner as the values returned by
    the time function.

    The original values of the tm_wday and tm_yday components of
    the structure are ignored, and the original values of the other
    components are not restricted to the ranges defined in <time.h>.
    Upon successful completion, the tm_wday and tm_yday components of
    the structure are set appropriately, and the other components are
    set to represent the specified time, with their values forced to
    the normal range.

    If the local time cannot be encoded, then mktime returns the
    value (time_t)(-1).

    The time_t type is defined in the <time.h> header file as
    follows:

    typedef unsigned long int time_t;

    Local time-zone information is set as if mktime called tzset.

    If the tm_isdst field in the local-time structure pointed to
    by timeptr is positive, mktime initially presumes that Daylight
    Savings Time (DST) is in effect for the specified time.

    If tm_isdst is 0, mktime initially presumes that DST is not in
    effect.

    If tm_isdst is negative, mktime attempts to determine whether or
    not DST is in effect for the specified time.