Q: I know that the library function localtime will convert a time_t into a broken-down struct tm, and that ctime will convert a time_t to a printable string. How can I perform the inverse operations of converting a struct tm or a string into a time_t?
A: ANSI C specifies a library function, mktime, which converts a struct tm to a time_t.
Converting a string to a time_t is harder, because of the wide variety of date and time formats which might be encountered. Some systems provide a strptime function, which is basically the inverse of strftime. Other popular functions are partime (widely distributed with the RCS package) and getdate (and a few others, from the C news distribution). See question 18.16.
K&R2 Sec. B10 p. 256
ISO Sec. 18.104.22.168
H&S Sec. 18.4 pp. 401-2