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localtime, _localtime32, _localtime64

  • Artikkeli

Converts a time value and corrects for the local time zone. More secure versions of these functions are available; see localtime_s, _localtime32_s, _localtime64_s.

Syntax

struct tm *localtime( const time_t *sourceTime );
struct tm *_localtime32( const __time32_t *sourceTime );
struct tm *_localtime64( const __time64_t *sourceTime );

Parameters

sourceTime
Pointer to stored time.

Return value

Return a pointer to the structure result, or NULL if the date passed to the function is:

  • Before midnight, January 1, 1970.

  • After 03:14:07, January 19, 2038, UTC (using _time32 and time32_t).

  • After 23:59:59, December 31, 3000, UTC (using _time64 and __time64_t).

_localtime64, which uses the __time64_t structure, allows dates to be expressed up through 23:59:59, December 31, 3000, coordinated universal time (UTC), whereas _localtime32 represents dates through 23:59:59 January 18, 2038, UTC.

localtime is an inline function that evaluates to _localtime64, and time_t is equivalent to __time64_t. If you need to force the compiler to interpret time_t as the old 32-bit time_t, you can define _USE_32BIT_TIME_T. _USE_32BIT_TIME_T causes localtime to evaluate to _localtime32. We don't recommend _USE_32BIT_TIME_T, because your application may fail after January 18, 2038, and it isn't allowed on 64-bit platforms.

The fields of the structure type tm store the following values, each of which is an int:

Field Description
tm_sec Seconds after minute (0 - 59).
tm_min Minutes after hour (0 - 59).
tm_hour Hours since midnight (0 - 23).
tm_mday Day of month (1 - 31).
tm_mon Month (0 - 11; January = 0).
tm_year Year (current year minus 1900).
tm_wday Day of week (0 - 6; Sunday = 0).
tm_yday Day of year (0 - 365; January 1 = 0).
tm_isdst Positive value if daylight saving time is in effect; 0 if daylight saving time isn't in effect; negative value if status of daylight saving time is unknown.

If the TZ environment variable is set, the C run-time library assumes rules appropriate to the United States for implementing the calculation of daylight-saving time (DST).

Remarks

The localtime function converts a time stored as a time_t value and stores the result in a structure of type tm. The long value sourceTime represents the seconds elapsed since midnight (00:00:00), January 1, 1970, UTC. This value is often obtained from the time function.

Both the 32-bit and 64-bit versions of gmtime, mktime, mkgmtime, and localtime all use a single tm structure per thread for the conversion. Each call to one of these routines destroys the result of the previous call.

localtime corrects for the local time zone if the user first sets the global environment variable TZ. When TZ is set, three other environment variables (_timezone, _daylight, and _tzname) are automatically set as well. If the TZ variable isn't set, localtime attempts to use the time zone information specified in the Date/Time application in Control Panel. If this information can't be obtained, PST8PDT, which signifies the Pacific Time Zone, is used by default. See _tzset for a description of these variables. TZ is a Microsoft extension and not part of the ANSI standard definition of localtime.

Note

The target environment should try to determine whether daylight saving time is in effect.

These functions validate their parameters. If sourceTime is a null pointer, or if the sourceTime value is negative, these functions invoke an invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, the functions return NULL and set errno to EINVAL.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Routine Required C header Required C++ header
localtime, _localtime32, _localtime64 <time.h> <ctime> or <time.h>

For more compatibility information, see Compatibility.

Example

// crt_localtime.cpp
// compile with: /W3
// This program uses _time64 to get the current time
// and then uses localtime64() to convert this time to a structure
// representing the local time. The program converts the result
// from a 24-hour clock to a 12-hour clock and determines the
// proper extension (AM or PM).

#include <stdio.h>
#include <string.h>
#include <time.h>

int main( void )
{
    struct tm *newtime;
    char am_pm[] = "AM";
    __time64_t long_time;

    _time64( &long_time );             // Get time as 64-bit integer.
                                       // Convert to local time.
    newtime = _localtime64( &long_time ); // C4996
    // Note: _localtime64 deprecated; consider _localetime64_s

    if( newtime->tm_hour > 12 )        // Set up extension.
        strcpy_s( am_pm, sizeof(am_pm), "PM" );
    if( newtime->tm_hour > 12 )        // Convert from 24-hour
        newtime->tm_hour -= 12;        //   to 12-hour clock.
    if( newtime->tm_hour == 0 )        // Set hour to 12 if midnight.
        newtime->tm_hour = 12;

    char buff[30];
    asctime_s( buff, sizeof(buff), newtime );
    printf( "%.19s %s\n", buff, am_pm );
}

Tue Feb 12 10:05:58 AM

See also

Time management
asctime, _wasctime
ctime, _ctime32, _ctime64, _wctime, _wctime32, _wctime64
_ftime, _ftime32, _ftime64
gmtime, _gmtime32, _gmtime64
localtime_s, _localtime32_s, _localtime64_s
time, _time32, _time64
_tzset