The <time.h> header provides C interfaces for calendar time: timestamps represented by time_t, human-oriented dates represented by struct tm, conversion, comparison, and formatting. It also includes interfaces for physical time measured in seconds and nanoseconds. section 8.6 section 8.9
Designated initialization of a calendar time
struct tm today = { .tm_year = 2019 - 1900, .tm_mon = 4 - 1, .tm_mday = 3, .tm_hour = 10, .tm_min = 0, .tm_sec = 47, }; printf("this year is %d, next year will be %d\n", today.tm_year + 1900, today.tm_year + 1900 + 1); Members such as tm_year and tm_mon use non-human-based offsets: tm_year counts from 1900 and tm_mon counts January as 0. tm_mday instead begins at 1. Unspecified members are initialized to zero. section 6.3.1
The principal calendar-time operations have distinct representations and responsibilities:
- time(0) obtains the current timestamp as time_t; difftime computes the difference between two time_t values. section 8.6
- struct tm stores seconds, minutes, hours, day, month, year, weekday, day-of-year, and daylight-saving status. section 8.6
- mktime normalizes calendar members, computes weekday and day-of-year values, updates daylight-saving status when requested, and returns time_t. section 8.6
time_t time(time_t *t);
double difftime(time_t time1, time_t time0);
time_t mktime(struct tm tm[1]);
size_t strftime(char s[static 1], size_t max,
char const format[static 1],
struct tm const tm[static 1]);
int timespec_get(struct timespec ts[static 1], int base);
int timespec_getres(struct timespec ts[static 1], int base);
Browse the core declarations for timestamp acquisition, interval calculation, calendar conversion, formatting, and timespec resolution. section 8.6
