asctime, ctime, gmtime, localtime, mktime - transform binary date and
time to ASCII
char *asctime(const struct tm *timeptr);
char *ctime(const time_t *timep);
struct tm *gmtime(const time_t *timep);
struct tm *localtime(const time_t *timep);
time_t mktime(struct tm *timeptr);
extern char *tzname;
long int timezone;
extern int daylight;
The ctime(), gmtime() and localtime() functions all take an argument of
data type time_t which represents calendar time. When interpreted as
an absolute time value, it represents the number of seconds elapsed
since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
The asctime() and mktime() functions both take an argument representing
broken-down time which is a binary representation separated into year,
month, day, etc. Broken-down time is stored in the structure tm which
is defined in <time.h> as follows:
int tm_sec; /* seconds */
int tm_min; /* minutes */
int tm_hour; /* hours */
int tm_mday; /* day of the month */
int tm_mon; /* month */
int tm_year; /* year */
int tm_wday; /* day of the week */
int tm_yday; /* day in the year */
int tm_isdst; /* daylight saving time */
The members of the tm structure are:
tm_sec The number of seconds after the minute, normally in the range 0
to 59, but can be up to 61 to allow for leap seconds.
tm_min The number of minutes after the hour, in the range 0 to 59.
The number of hours past midnight, in the range 0 to 23.
The day of the month, in the range 1 to 31.
tm_mon The number of months since January, in the range 0 to 11.
The number of years since 1900.
The number of days since Sunday, in the range 0 to 6.
The number of days since January 1, in the range 0 to 365.
A flag that indicates whether daylight saving time is 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.
The ctime() function converts the calendar time timep into a string of
"Wed Jun 30 21:49:08 1993\n"
The abbreviations for the days of the week are `Sun', `Mon', `Tue',
`Wed', `Thu', `Fri', and `Sat'. The abbreviations for the months are
`Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct',
`Nov', and `Dec'. The return value points to a statically allocated
string which might be overwritten by subsequent calls to any of the
date and time functions. The function also sets the external variable
tzname with information about the current time zone.
The gmtime() function converts the calendar time timep to broken-down
time representation, expressed in Coordinated Universal Time (UTC).
The localtime() function converts the calendar time timep to brokentime
representation, expressed relative to the user's specified time
zone. The function sets the external variables tzname with information
about the current time zone, timezone with the difference between
Coordinated Universal Time (UTC) and local standard time in seconds,
and daylight to a non-zero value if standard US daylight savings time
The asctime() function converts the broken-down time value timeptr into
a string with the same format as ctime(). The return value points to a
statically allocated string which might be overwritten by subsequent
calls to any of the date and time functions.
The mktime() function converts a broken-down time structure, expressed
as local time, to calendar time representation. The function ignores
the specified contents of the structure members tm_wday and tm_yday and
recomputes them from the other information in the broken-down time
structure. If structure members are outside their legal interval, they
will be normalized (so that, e.g., 40 October is changed into 9 November).
Calling mktime() also sets the external variable tzname with
information about the current time zone. If the specified broken-down
time cannot be represented as calendar time (seconds since the epoch),
mktime() returns a value of (time_t)(-1) and does not alter the tm_wday
and tm_yday members of the broken-down time structure.
SVID 3, POSIX, BSD 4.3, ISO 9899
date(1), gettimeofday(2), time(2), tzset(3), difftime(3), strftime(3).
BSD 1996-04-26 CTIME(3)
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