The header time.h defines four macros, and declares several types and functions for manipulating time. Many functions deal with a calender time that represents the current date (according to the Gregorian calender) and time. Some functions deal with local time, which is the calender time expressed for some specific time zone, and with Daylight Saving Time, which is a temporary change in the algorithm for determining local time. The local timezone and Daylight Saving Time are implementation-defined.
Chapter 1.: Date and Time Functions -- <time.h>