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Standard C Time & Date functions

Standard C Time & Date

This section will cover the Time and Date elements of the C Standard Library.

#include <ctime>
char *asctime( const struct tm *ptr );

The function asctime() converts the time in the struct ‘ptr’ to a character string of the following format:

day month date hours:minutes:seconds year

An example:

Mon Jun 26 12:03:53 2000


#include <ctime>
clock_t clock( void );

The clock() function returns the processor time since the program started, or -1 if that information is unavailable. To convert the return value to seconds, divide it by CLOCKS_PER_SEC.

If your compiler and library is POSIX compliant, then CLOCKS_PER_SEC is always defined as 1000000.


#include <ctime>
char *ctime( const time_t *time );

The ctime() function converts the calendar time time to local time of the format:

day month date hours:minutes:seconds year            

using ctime() is equivalent to

asctime( localtime( tp ) );


#include <ctime>
double difftime( time_t time2, time_t time1 );

The function difftime() returns time2time1, in seconds.


#include <ctime>
struct tm *gmtime( const time_t *time );

The gmtime() function returns the given time in Coordinated Universal Time (usually Greenwich mean time), unless it’s not supported by the system, in which case NULL is returned. Watch out for the static return.


#include <ctime>
struct tm *localtime( const time_t *time );

The function localtime() converts calendar time time into local time. Watch out for the static return.


#include <ctime>
time_t mktime( struct tm *time );

The mktime() function converts the local time in time to calendar time, and returns it. If there is an error, -1 is returned.


#include <clocale>
char *setlocale( int category, const char * locale );

The setlocale() function is used to set and retrieve the current locale. If locale is NULL, the current locale is returned. Otherwise, locale is used to set the locale for the given category.

category can have the following values:

Value Description
LC_ALL All of the locale
LC_TIME Date and time formatting
LC_NUMERIC Number formatting
LC_COLLATE String collation and regular expression matching
LC_CTYPE Regular expression matching, conversion, case-sensitive comparison, wide character functions, and character classification.
LC_MONETARY For monetary formatting
LC_MESSAGES For natural language messages


#include <ctime>
size_t strftime( char *str, size_t maxsize, const char *fmt, struct tm *time );

The function strftime() formats date and time information from time to a format specified by fmt, then stores the result in str (up to maxsize characters). Certain codes may be used in fmt to specify different types of time:

Code Meaning
 %a abbreviated weekday name (e.g. Fri)
 %A full weekday name (e.g. Friday)
 %b abbreviated month name (e.g. Oct)
 %B full month name (e.g. October)
 %c the standard date and time string
 %d day of the month, as a number (1-31)
 %H hour, 24 hour format (0-23)
 %I hour, 12 hour format (1-12)
 %j day of the year, as a number (1-366)
 %m month as a number (1-12).
 %M minute as a number (0-59)
 %p locale’s equivalent of AM or PM
 %S second as a number (0-59)
 %U week of the year, (0-53), where week 1 has the first Sunday
 %w weekday as a decimal (0-6), where Sunday is 0
 %W week of the year, (0-53), where week 1 has the first Monday
 %x standard date string
 %X standard time string
 %y year in decimal, without the century (0-99)
 %Y year in decimal, with the century
 %Z time zone name
 %% a percent sign

Some versions of Microsoft Visual C++ may use values that range from 0-11 to describe %m (month as a number).


#include <ctime>
time_t time( time_t *time );

The function time() returns the current time, or -1 if there is an error. If the argument time is given, then the current time is stored in time.

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