struct tm
{
  int tm_sec; /*seconds after the minute (0-61)*/
  int tm_min; /*minutes after the hour (0-59)*/
  int tm_hour; /*hours since midnight (0-23)*/
  int tm_mday; /*day of the month (1-31)*/
  int tm_mon; /* months since January  (0-11)*/
  int tm_year; /* elapsed years since 1900 */
  int tm_wday; /* days since Sunday (0-6)*/
  int tm_yday; /*days since January 1st  (0-365)*/
  int tm_isdst; /*1 if daylight savings is on, zero if not,
                -1 if unknown*/
};

Remember: the key to manipulating date and time is knowing how to convert time_t to tm and vice versa. To fill a tm object with the local time, use the function localtime():

struct tm* localtime (const time_t *pt);

localtime() takes a pointer to a valid time_t object, converts it to a local static tm struct and returns its address. Note that subsequent invocations of localtime() override the previous value of its local static tm object. Therefore, you should copy the result immediately to your own tm object. The following program fills a tm object with the current local time:

#include <ctime>
using namespace std;
int main()
{
 time_t curr;
 tm local;
 time(&curr); // get current time_t value
 local=*(localtime(&curr)); // dereference and assign
}

To convert a tm struct to time_t, use the mktime() function:

time_t curr;
tm local;
time(&curr); // get current time_t value
local=*(localtime(&curr)); // dereference and assign
time_t temp=mktime(&local); // temp and curr are equal
 

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Next: Time Differences and Time Zones

Introduction

Breaking It into Pieces

Time Differences and Time Zones