2.15.1 Variables and Definitions

NULL is the value of a null pointer constant.
CLOCKS_PER_SEC is the number of processor clocks per second.
size_t is the unsigned integer result of the sizeof keyword.
clock_t is a type suitable for storing the processor time.
time_t is a type suitable for storing the calendar time.

struct tm is a structure used to hold the time and date. Its members are as follows:

     int tm_sec;    /* seconds after the minute (0 to 61) */
     int tm_min;    /* minutes after the hour (0 to 59) */
     int tm_hour;   /* hours since midnight (0 to 23) */
     int tm_mday;   /* day of the month (1 to 31) */
     int tm_mon;    /* months since January (0 to 11) */
     int tm_year;   /* years since 1900 */
     int tm_wday;   /* days since Sunday (0 to 6 Sunday=0) */
     int tm_yday;   /* days since January 1 (0 to 365) */
     int tm_isdst;  /* Daylight Savings Time */

If tm_isdst is zero, then Daylight Savings Time is not in effect. If it is a positive value, then Daylight Savings Time is in effect. If it is negative, then the function using it is requested to attempt to calculate whether or not Daylight Savings Time is in effect for the given time.

Note that tm_sec may go as high as 61 to allow for up to two leap seconds.