Convert tm structure to time_t value.
  Checks the members of the tm structure passed as parameter ptm adjusting the values if the ones provided are not in the possible range or they are incomplete or mistaken and then translates that structure to a time_t value (seconds elapsed since Jan 1, 1970) that is returned.
  The original values of tm_wday and tm_yday members of ptm are ignored and filled with the correspondent ones to the calculated date. The range of tm_mday is not checked until tm_mon and tm_year are determined.

Parameters.

ptm

Pointer to a tm structure, that contains data to be computed.

Return Value.
  A time_t value corresponding to the date and time passed in ptm parameter.
  On error, a -1 value is returned.

Portability.
  Defined in ANSI-C.

Example.

/* mktime example: weekday calculator */
#include <stdio.h>
#include <time.h>

main ()
{
  time_t rawtime;
  struct tm * timeinfo;
  int year, month ,day;
  char * weekday[] = { "Sunday", "Monday",
                       "Tuesday", "Wednesday",
                       "Thursday", "Friday", "Saturday"};

  /* prompt user for date */
  printf ("Enter year: "); scanf ("%d",&year);
  printf ("Enter month: "); scanf ("%d",&month);
  printf ("Enter day: "); scanf ("%d",&day);

  /* get current timeinfo and modify it to user's choice */
  time ( &rawtime );
  timeinfo = localtime ( &rawtime );
  timeinfo->tm_year = year - 1900;
  timeinfo->tm_mon = month - 1;
  timeinfo->tm_mday = day;

  /* call mktime: timeinfo->tm_wday will be set */
  mktime ( timeinfo );

  printf ("That day is a %s.\n", weekday[timeinfo->tm_wday]);
  
  return 0;
}

See also.
  asctime, gmtime, localtime, time