struct tm represents calendar time as named components: seconds, minutes, hours, day, month, year, weekday, day of year, and daylight-saving status. Named members avoid the ambiguity of storing date parts in array positions, and expressions such as today.tm_year access each component directly. section 6.3.1 section 8.6
struct tm {
int tm_sec; // Seconds after the minute [0, 60]
int tm_min; // Minutes after the hour [0, 59]
int tm_hour; // Hours since midnight [0, 23]
int tm_mday; // Day of the month [1, 31]
int tm_mon; // Months since January [0, 11]
int tm_year; // Years since 1900
int tm_wday; // Days since Sunday [0, 6]
int tm_yday; // Days since January [0, 365]
int tm_isdst;// Daylight Saving Time flag
};
The member declarations make the representation and its principal ranges explicit. section 6.3.1
Designated initialization
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); section 6.3.1
Interpret the fields carefully:
- tm_mon starts at 0, so January is 0; tm_wday likewise starts at 0, with Sunday represented by 0. section 8.6
- tm_mday starts at 1, tm_year stores the Gregorian year minus 1900, and tm_sec ranges from 0 through 60 for leap seconds. section 8.6
- Omitted members in a struct initializer become zero, but zero does not necessarily make supplemental fields consistent with the date. section 6.3.1
- mktime normalizes the hierarchical fields, computes tm_wday and tm_yday, updates an indeterminate tm_isdst flag, and returns a time_t representation. section 8.6
