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 representation
separated into year, month, day, etc.
Broken-down time is stored
in the structure tm which is defined in <time.h> as follows:
struct tm {
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 60 to allow for leap seconds.
tm_min
The number of minutes after the hour, in the range 0 to 59.
tm_hour
The number of hours past midnight, in the range 0 to 23.
tm_mday
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.
tm_year
The number of years since 1900.
tm_wday
The number of days since Sunday, in the range 0 to 6.
tm_yday
The number of days since January 1, in the range 0 to 365.
tm_isdst
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 call
ctime(t) is equivalent to
asctime(localtime(t)). It converts the calendar time t into a string of the form
"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 (see
tzset(3))
with information about the current time zone.
The re-entrant version ctime_r() does the same, but stores the
string in a user-supplied buffer of length at least 26. It need not
set tzname.
The gmtime() function converts the calendar time timep to
broken-down time representation, expressed in Coordinated Universal Time
(UTC). It may return NULL when the year does not fit into an integer.
The return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions.
The gmtime_r() function does the same, but stores the data in a
user-supplied struct.
The localtime() function converts the calendar time timep to
broken-time representation, expressed relative to the user’s specified
time zone. The function acts as if it called
tzset(3)
and 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 daylight savings
time rules apply during some part of the year.
The return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions.
The localtime_r() function does the same, but stores the data in a
user-supplied struct. It need not set tzname.
The asctime() function converts the broken-down time value
tm 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 asctime_r() function does the same, but stores the string in
a user-supplied buffer of length at least 26.
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.
The four functions
asctime(),
ctime(),
gmtime() and
localtime() return a pointer to static data and hence are not thread-safe.
Thread-safe versions
asctime_r(),
ctime_r(),
gmtime_r() and
localtime_r() are specified by SUSv2, and available since libc 5.2.5.
In many implementations, including
glibc, a 0 in
tm_mday is interpreted as meaning the last day of the preceding month.
The glibc version of struct tm has additional fields
long tm_gmtoff; /* Seconds east of UTC */
const char *tm_zone; /* Timezone abbreviation */
defined when _BSD_SOURCE was set before including
<time.h>. This is a BSD extension, present in 4.3BSD-Reno.
