#### 21.4.3 Broken-down Time

Calendar time is represented by the usual GNU C Library functions as an elapsed time since a fixed base calendar time. 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. Broken-down time values are not useful for calculations, but 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.
> `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 UTC. For example, for U.S. Eastern Standard Time, the value is `-5*60*60`. The `tm_gmtoff` field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment.
> `const char *tm_zone`
>   This field is the name for the time zone that was used to compute this broken-down time value. Like `tm_gmtoff`, this field is a BSD and GNU extension, and is not visible in a strict ISO C environment.

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

> 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 `ctime`, `gmtime`, or `localtime`. (But no other library function overwrites the contents of this object.)

> 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` has one other effect: it sets the variable `tzname` with information about the current time zone. See Time Zone Functions.

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. POSIX.1c introduced a variant of this function.

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

> 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.

> 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)

> This function is similar to `localtime`, except that the broken-down time is expressed as Coordinated Universal Time (UTC) (formerly called Greenwich Mean Time (GMT)) rather than relative to a local time zone.

As for the `localtime` function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for `gmtime`.

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

> This function is similar to `localtime_r`, except that it converts just like `gmtime` the given time as Coordinated Universal Time.

> 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)

> The `mktime` function is used to convert a broken-down time structure to a simple time representation. It also “normalizes” the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

> The `mktime` function ignores the specified contents of the `tm_wday` and `tm_yday` 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 `tm_wday` and `tm_yday`).

> 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 variable `tzname` with information about the current time zone. See Time Zone Functions.

— Function: time_t timelocal (struct tm *brokentime)

> `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)

> `timegm` is functionally identical to `mktime` except it always takes the input values to be Coordinated Universal Time (UTC) regardless of any local time zone setting.

> Note that `timegm` is the inverse of `gmtime`.

> Portability note: `mktime` is essentially universally available. `timegm` is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the `TZ` environment variable to UTC, call `mktime`, then set `TZ` back.