The <time.h> header declares functions that allow us to work with date/time. In this appendix, we explain how to obtain and format the current time and date.
The
time function is declared inside the
<time.h> header and returns the current date-time (date-time since epoch) as an object of type
time_t. The function has the following signature:
timet_ time(time_t *arg);
The type
time_t is a type capable of storing times. The time function can return the calendar time when arg is NULL:
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t mytime = time(NULL);
printf("Obtained the current time to a mytime variable.
");
}
or store it inside an object pointed to by
arg
:
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t mytime;
time(&mytime);
printf("Obtained the current time to a mytime variable.
");
}
There are a number of steps involved when getting and formatting the time.
Get the current date-time using a time function.
Store/convert the obtained date-time into a tm struct using localtime or gmtime.
Format the obtained time using the strftime.
The following example obtains a date-time and stores it into a
tm struct using a
localtime function
:
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t mytime = time(NULL);
struct tm *now;
now = localtime(&mytime);
printf("Obtained and stored the current time.
");
}
The
localtime function converts obtained local time to a
tm calendar time. The
tm structure holds the calendar date and time. The
tm structure
has the following predefined member fields of type
int:tm_sec – seconds from 0 to 60
tm_min – minutes from 0 to 59
tm_hour – hours from 0 to 23
tm_mday – days from 1 to 31
tm_mon – months from 0 to 11
tm_year – years since 1900
tm_wday – days since Sunday from 0 to 6
tm_yday – days since January the 1st from 0 to 365
tm_isdst – daytime saving value, positive if active, zero if not
The final thing left to do is to convert the
tm time to a string using a
strftime function
and appropriate format specifiers:
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t mytime = time(NULL);
struct tm *nowtm;
char str[70];
nowtm = localtime(&mytime);
strftime(str, sizeof str, "%T", nowtm);
printf("The time is: %s
", str);
}
The strftime function
converts the calendar date/time stored inside the tm structure to a string according to format specifiers used. Here we used the %T format specifier, which is the same as the %H:%M:%S format.
To format the obtained date/time as a date only, we can use the
%D format specifier. Example:
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t mytime = time(NULL);
struct tm *nowtm;
char str[70];
nowtm = localtime(&mytime);
strftime(str, sizeof str, "%D", nowtm);
printf("The date is: %s
", str);
}
This example uses the %D format specifier inside the strftime function to output only the date part of the obtained date-time. The %D format specifier is equivalent to %m/%d/%y format.
When we populate the
tm structure
, we can access its individual fields. For example, if we need to access and display minutes and seconds as integers, we write:
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t mytime = time(NULL);
struct tm *nowtm;
nowtm = localtime(&mytime);
printf("Minutes and seconds are: %d:%d
", nowtm->tm_min, nowtm->tm_sec);
}
Output:Minutes and seconds are: 42:12
In this example, we do not convert the obtained date-time to a string strftime function. We simply use the tm structure’s fields representing minutes and numbers, called tm_min and tm_sec, and print them out using the printf function.
