std::mktime (3) - Linux Manuals
Defined in header <ctime>
std::time_t mktime( std::tm* time );
Converts local calendar time to a time since epoch as a time_t object. time->tm_wday and time->tm_yday are ignored. The values in time are permitted to be outside their normal ranges.
A negative value of time->tm_isdst causes mktime to attempt to determine if Daylight Saving Time was in effect.
If the conversion is successful, the time object is modified. All fields of time are updated to fit their proper ranges. time->tm_wday and time->tm_yday are recalculated using information available in other fields.
time - pointer to a std::tm object specifying local calendar time to convert
Time since epoch as a std::time_t object on success or -1 if time cannot be represented as a std::time_t object.
If the std::tm object was obtained from std::get_time or the POSIX strptime, the value of tm_isdst is indeterminate, and needs to be set explicitly before calling mktime.
Display the time 100 months ago.
// Run this code