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22.5.3 Broken-down Time ¶

Simple calendar times represent absolute times as elapsed times since an epoch. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, broken-down time is a binary representation of calendar time separated into year, month, day, and so on. Although broken-down time values are painful to calculate with, they are useful for printing human readable time information.

A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.

The symbols in this section are declared in the header file time.h.

Data Type: struct tm ¶

This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order.

int tm_sec

This is the number of full seconds since the top of the minute (normally in the range 0 through 59, but the actual upper limit is 60, to allow for leap seconds if leap second support is available).

int tm_min

This is the number of full minutes since the top of the hour (in the range 0 through 59).

int tm_hour

This is the number of full hours past midnight (in the range 0 through 23).

int tm_mday

This is the ordinal day of the month (in the range 1 through 31). Watch out for this one! As the only ordinal number in the structure, it is inconsistent with the rest of the structure.

int tm_mon

This is the number of full calendar months since the beginning of the year (in the range 0 through 11). Watch out for this one! People usually use ordinal numbers for month-of-year (where January = 1).

int tm_year

This is the number of full calendar years since 1900.

int tm_wday

This is the number of full days since Sunday (in the range 0 through 6).

int tm_yday

This is the number of full days since the beginning of the year (in the range 0 through 365).

int tm_isdst ¶

This is a flag that indicates whether daylight saving time is (or was, or will be) in effect at the time described. The value is positive if daylight saving time is in effect, zero if it is not, and negative if the information is not available. Although this flag is useful when passing a broken-down time to the mktime function, for other uses this flag should be ignored and the tm_gmtoff and tm_zone fields should be inspected instead.

long int tm_gmtoff

This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of the Prime Meridian. For example, for U.S. Eastern Standard Time, the value is -5*60*60.

const char *tm_zone

This field is the abbreviation for the time zone that was used to compute this broken-down time value.

Portability note: The tm_gmtoff and tm_zone fields are derived from BSD and are POSIX extensions to ISO C. Code intended to be portable to operating systems that lack these fields can instead use time zone state variables, although those variables are unreliable when the TZ environment variable has a geographical format. See State Variables for Time Zones.

Function: struct tm * localtime (const time_t *time) ¶

Preliminary: | MT-Unsafe race:tmbuf env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

The localtime function converts the simple time pointed to by time to broken-down time representation, expressed relative to the user’s specified time zone.

The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to gmtime or localtime. (No other library function overwrites the contents of this object.) In the GNU C Library, the structure’s tm_zone points to a string with a storage lifetime that lasts indefinitely; on other platforms, the lifetime may expire when the TZ environment variable is changed.

The return value is the null pointer if time cannot be represented as a broken-down time; typically this is because the year cannot fit into an int.

Calling localtime also sets the time zone state as if tzset were called. See State Variables for Time Zones.

Using the localtime function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. A variant function avoids this problem.

Function: struct tm * localtime_r (const time_t *time, struct tm *resultp) ¶

Preliminary: | MT-Safe env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

The localtime_r function works just like the localtime function. It takes a pointer to a variable containing a simple time and converts it to the broken-down time format.

But the result is not placed in a static buffer. Instead it is placed in the object of type struct tm to which the parameter resultp points. Also, the time zone state is not necessarily set as if tzset were called.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

Function: struct tm * gmtime (const time_t *time) ¶

Preliminary: | MT-Unsafe race:tmbuf env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

This function is similar to localtime, except that the broken-down time is expressed as UTC rather than relative to a local time zone. The broken-down time’s tm_gmtoff is 0, and its tm_zone is a string "UTC" with static storage duration.

As for the localtime function we have the problem that the result is placed in a static variable. A thread-safe replacement is also provided for gmtime.

Function: struct tm * gmtime_r (const time_t *time, struct tm *resultp) ¶

Preliminary: | MT-Safe env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

This function is similar to localtime_r, except that it converts just like gmtime the given time as UTC.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

Function: time_t mktime (struct tm *brokentime) ¶

Preliminary: | MT-Safe env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

The mktime function converts a broken-down time structure to a simple time representation. It also normalizes the contents of the broken-down time structure, and fills in some components based on the values of the others.

The mktime function ignores the specified contents of the tm_wday, tm_yday, tm_gmtoff, and tm_zone members of the broken-down time structure. It uses the values of the other components to determine the calendar time; it’s permissible for these components to have unnormalized values outside their normal ranges. The last thing that mktime does is adjust the components of the brokentime structure, including the members that were initially ignored.

If the specified broken-down time cannot be represented as a simple time, mktime returns a value of (time_t)(-1) and does not modify the contents of brokentime.

Calling mktime also sets the time zone state as if tzset were called; mktime uses this information instead of brokentime’s initial tm_gmtoff and tm_zone members. See State Variables for Time Zones.

Function: time_t timelocal (struct tm *brokentime) ¶

Preliminary: | MT-Safe env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

timelocal is functionally identical to mktime, but more mnemonically named. Note that it is the inverse of the localtime function.

Portability note: mktime is essentially universally available. timelocal is rather rare.

Function: time_t timegm (struct tm *brokentime) ¶

Preliminary: | MT-Safe env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.

timegm is functionally identical to mktime except it always takes the input values to be UTC regardless of any local time zone setting.

Note that timegm is the inverse of gmtime.

Portability note: mktime is essentially universally available. Although timegm is standardized by C23, some other systems lack it; to be portable to them, you can set the TZ environment variable to UTC, call mktime, then set TZ back.