NAME

ctime, localtime, gmtime, asctime, tzset − convert date and time to string

SYNOPSIS

#include <time.h>

char ∗ctime (clock)
long ∗clock;

struct tm ∗localtime (clock)
long ∗clock;

struct tm ∗gmtime (clock)
long ∗clock;

char ∗asctime (tm)
struct tm ∗tm;

extern long timezone;

extern int daylight;

extern char ∗tzname[2];

void tzset ( )

DESCRIPTION

Ctime converts a long integer, pointed to by clock, representing the time in seconds since 00:00:00 GMT, January 1, 1970, and returns a pointer to a 26-character string in the following form. All the fields have constant width.

Sun Sep 16 01:03:52 1973\n\0

Localtime and gmtime return pointers to “tm” structures, described below.  Localtime corrects for the time zone and possible Daylight Savings Time; gmtime converts directly to Greenwich Mean Time (GMT), which is the time the UNIX System uses. 

Asctime converts a “tm” structure to a 26-character string, as shown in the above example, and returns a pointer to the string. 

Declarations of all the functions and externals, and the “tm” structure, are in the <time.h> header file.  The structure declaration is:

structtm {
int tm_sec;/∗ seconds (0 - 59) ∗/
int tm_min;/∗ minutes (0 - 59) ∗/
int tm_hour;/∗ hours (0 - 23) ∗/
int tm_mday;/∗ day of month (1 - 31) ∗/
int tm_mon;/∗ month of year (0 - 11) ∗/
int tm_year;/∗ year − 1900 ∗/
int tm_wday;/∗ day of week (Sunday = 0) ∗/
int tm_yday;/∗ day of year (0 - 365) ∗/
int tm_isdst;
};

Tm_isdst is non-zero if Daylight Savings Time is in effect. 

The external long variable timezone contains the difference, in seconds, between GMT and local standard time (in EST, timezone is 5∗60∗60); the external variable daylight is non-zero if and only if the standard U.S.A.  Daylight Savings Time conversion should be applied.  The program knows about the peculiarities of this conversion in 1974 and 1975; if necessary, a table for these years can be extended. 

If an environment variable named TZ is present, asctime uses the contents of the variable to override the default time zone.  The value of TZ must be a three-letter time zone name, followed by a number representing the difference between local time and Greenwich Mean Time in hours, followed by an optional three-letter name for a daylight time zone.  For example, the setting for New Jersey would be EST5EDT.  The effects of setting TZ are thus to change the values of the external variables timezone and daylight; in addition, the time zone names contained in the external variable

 char ∗tzname[2] = { "EST", "EDT" };

are set from the environment variable TZ.  The function tzset sets these external variables from TZ; tzset is called by asctime and may also be called explicitly by the user. 

Note that in most installations, TZ is set by default when the user logs on, to a value in the local /etc/TZ file (see profile(4)).

SEE ALSO

time(2), getenv(3C), profile(4), environ(5). 

BUGS

The return values point to static data whose content is overwritten by each call. 

Version 2.4  —  May 08, 1986