Time and Date functions

Time and Date functions

The C library offers a lot of functions for working with dates and time. The first of them is the time function that returns the number of seconds that have passed since January first 1970, at midnight.

Several structures are defined that hold time information. The most important from them are 151e45b the "tm" structure and the "timeb" structure.

struct tm

The fields are self-explanatory. The structure "timeb" is defined in the directory include\sys, as follows:

struct timeb ;

We show here a small program that displays the different time settings.

#include <time.h>

#include <stdio.h>

#include <sys/types.h>

#include <sys/timeb.h>

#include <string.h>

void main()

/* Display operating system-style date and time.

strtime( tmpbuf );

printf( "OS time:\t\t\t\t%s\n", tmpbuf );

strdate( tmpbuf );

printf( "OS date:\t\t\t\t%s\n", tmpbuf );

/* Get UNIX-style time and display as number and string. */

time( &ltime );

printf( "Time in seconds since UTC 1/1/70:\t%ld\n", ltime );

printf( "UNIX time and date:\t\t\t%s", ctime( &ltime ) );

/* Display UTC. See note (1) in text */

gmt = gmtime( &ltime );

printf( "Coordinated universal time:\t\t%s", asctime( gmt ) );

/* Convert to time structure and adjust for PM if necessary. */

today = localtime( &ltime );

if( today->tm_hour > 12 )

if( today->tm_hour == 0 ) /* Adjust if midnight hour. */

today->tm_hour = 12;

/* See note (2) in text */

printf( "12-hour time:\t\t\t\t%.8s %s\n",

asctime( today ) + 11, ampm );

/* Print additional time information.

ftime( &tstruct );

printf( "Plus milliseconds:\t\t\t%u\n", tstruct.millitm );

printf( "Zone difference in seconds from UTC:\t%u\n",

tstruct.timezone );

printf( "Time zone name:\t\t\t\t%s\n", _tzname[0] );

printf( "Daylight savings:\t\t\t%s\n", // See note (3) in text

tstruct.dstflag ? "YES" : "NO" );

/* Make time for noon on Christmas, 1993. */

if( mktime( &xmas ) != (time_t)-1 )

printf( "Christmas\t\t\t\t%s\n", asctime( &xmas ) );

/* Use time structure to build a customized time string. */

today = localtime( &ltime );

/* Use strftime to build a customized time string. */

strftime( tmpbuf, 128,

"Today is %A, day %d of the month of %B in the year %Y.\n",

today );

printf( tmpbuf );

We use this opportunity for introducing new C constructs.

We see the function call gmtime(&ltime);. What does this mean? The function gmtime requires a pointer to a variable of type time_t. We do not have a pointer, so we make one "on the fly" by using the "address-of" operator.

The printf statement uses pointer addition to skip the first 11 characters of the result of asctime. That function returns a pointer to a character string. To skip characters we just add up a constant (11) to that pointer, effectively skipping those characters. Since we want to display the 8 following characters only, we pass a width argument to the %s directive of printf. As you know, "%s" is a directive that instructs printf to display a character string. Since we give it a maximum width of 8, only the first 8 chars will be displayed.

We see here the construct (expr) ? val1 : val2; This construct evaluates first the expression, in this case "tstruct.dstflag". If the value is different than zero, the return value of the expression will be the first value, in this case the character string "YES". If the expression evaluates to zero, the second value will be choosen , in this case the character string "NO". The result of this is passed to printf as an argument.

The Windows system too has a lot of time-related functions. Here is a handy list of the most important. Note that file times are kept using 64 bits in modern versions of windows, i.e. the numbers represent the number of 100 nanosecond intervals since January first, 1601.