4.271.11 time_t mktime
time_t mktime(struct tm *timeptr)
This function 'compiles' the elements of a broken-down time structure, returning a binary time stamp. The elements of timeptr are interpreted as representing Local Time.
The original values of the tm_wday and tm_yday elements of the structure are ignored, and the original values of the other elements are not restricted to the ranges stated for struct tm.
On successful completion, the values of all elements of timeptr are set to the appropriate range.