How do I use the C date and time functions on UNIX?
Terminology
A date/time can be in two formats:
- calendar time (a.k.a. simpletime) – time as an absolute value typically since some base time, often referred to as the Coordinated Universal Time
- localtime (a.k.a. broken-down time) – a calendar time made up of components of year, month, day etc. which takes into account the local time zone including Daylight Saving Time if applicable.
Data types
The date/time functions and types are declared in the time.h header file.
Time can be stored as a whole number or as an instance of a structure:
as a number using the time_t arithmetic type – to store calendar time as the number of seconds elapsed since the UNIX epoch January 1, 1970 00:00:00
using the structure timeval – to store calendar time as the number of seconds and nanoseconds elapsed since the UNIX epoch January 1, 1970 00:00:00
using the structure tm to store localtime, it contains attributes such as the following:
tm_hour tm_min tm_isdst
The tm_isdst attribute above is used to indicate Daylight Saving Time (DST). If the value is positive it is DST, if the value is 0 it is not DST.
Program to print the current Coordinated Universal Time
#include <stdio.h>#include <time.h>int main ( int argc, char *argv[] ){ time_t now; now = time ( NULL ); printf ( "It’s %ld seconds since January 1, 1970 00:00:00", (long) now ); return 0;}In the program above the function time reads the UNIX system time, subtracts that from January 1, 1970 00:00:00 (the UNIX epoch) and returns its result in seconds.
Program to print the current local time
#include <stdio.h>#include <time.h>int main ( int argc, char *argv[] ){ time_t now; struct tm *lcltime; now = time ( NULL ); lcltime = localtime ( &now ); printf ( "The time is %d:%d\n", lcltime->tm_hour, lcltime->tm_min ); return 0;}In the program above the function localtime converts the elapsed time in seconds from the UNIX epoch into the broken-down time. localtime reads the UNIX environment TZ (through a call to the tzset function) to return the time relative to the timezone and to set the tm_isdst attribute.
A typical setting of the TZ variable in UNIX (using bash) would be as follows:
export TZ=GMTor
export TZ=US/EasternProgram to print the current formatted Greenwich Mean Time
#include <stdio.h>#include <time.h>int main ( int argc, char *argv[] ){ time_t now; struct tm *gmt; char formatted_gmt [50]; now = time ( NULL ); gmt = gmtime ( &now ); strftime ( formatted_gmt, sizeof(formatted_gmt), "%I:%M %p", gmt ); printf ( "The time is %s\n", formatted_gmt ); return 0;}In the program above the function strftime provides specialised formatting of dates.