Code:
NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r, localtime_r - transform date and
time to broken-down time or ASCII
SYNOPSIS
#include <time.h>
char *asctime(const struct tm *tm);
char *asctime_r(const struct tm *tm, char *buf);
char *ctime(const time_t *timep);
char *ctime_r(const time_t *timep, char *buf);
struct tm *gmtime(const time_t *timep);
struct tm *gmtime_r(const time_t *timep, struct tm *result);
struct tm *localtime(const time_t *timep);
struct tm *localtime_r(const time_t *timep, struct tm *result);
time_t mktime(struct tm *tm);
DESCRIPTION
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 rep-
resentation 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 61 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 informa-
tion 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 rela-
tive to the user's specified time zone. The function acts as if it called tzset(3) and sets the exter-
nal 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 subse-
quent 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.
RETURN VALUE
Each of these functions returns the value described, or NULL (-1 in case of mktime()) in case an error
was detected.
NOTES
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 spec-
ified 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.
CONFORMING TO
SVID 3, POSIX, 4.3BSD, ISO 9899
SEE ALSO
date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3), strftime(3), strptime(3), tzset(3)
hope dat helps 
here's wat i got from localtime manual page
