C++ mktime()

The mktime() function is defined in <ctime> header file.

mktime() prototype

time_t mktime(tm* time);

The mktime function takes a pointer to a tm object as its argument and returns the time since epoch as a value of type time_t. The values, time->tm_wday and time->tm_yday are ignored.

If the value of time->tm_isdst is negative, it causes mktime to attempt to determine if Daylight Saving Time was in effect.

mktime() Parameters

• time: A pointer to a tm object that represents the local calendar time to convert.

mktime() Return value

• On success, the mktime() function returns the time since epoch as an object of type time_t.
• On failure it returns -1.

Example: How mktime() function works?

#include <iostream>
#include <ctime>
using namespace std;

int main ()
{
	time_t tim;
	tm *ptr;

	int y = 2017, m = 4, d = 20;
	char weekday[7][20] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
	
	time(&tim);
	ptr = localtime(&tim);

	// tm_year is time since 1900
	ptr->tm_year = y - 1900;
	ptr->tm_mon = m - 1;
	ptr->tm_mday = d;
	
	mktime (ptr);
	cout << "April 20, 2017 was " << weekday[ptr->tm_wday];
	
	return 0;
}

When you run the program, the output will be:

April 4, 2017 was a Thursday