C++ Cookbook

5.1. Obtaining the Current Date and Time

Problem

You want to retrieve the current date and time from the user’s computer, either as a local time or as a Coordinated Universal Time (UTC).

Solution

Call the time function from the <ctime> header, passing a value of 0 as the parameter. The result will be a time_t value. You can use the gmtime function to convert the time_t value to a tm structure representing the current UTC time (a.k.a. Greenwich Mean Time or GMT); or, you can use the localtime function to convert the time_t value to a tm structure representing the local time. The program in Example 5-1 obtains the current date/time, and then converts it to local time and outputs it. Next, the program converts the current date/time to a UTC date/time and outputs that.

Example 5-1. Getting the local and UTC times

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

int main()
{
  // Current date/time based on current system
  time_t now = time(0);
   
  // Convert now to tm struct for local timezone
  tm* localtm = localtime(&now);
  cout << "The local date and time is: " << asctime(localtm) << endl;

  // Convert now to tm struct for UTC
  tm* gmtm = gmtime(&now);
  if (gmtm != NULL) {
     cout << "The UTC date and time is: " << asctime(gmtm) << endl;
  }
  else {
    cerr << "Failed to get the UTC date and time" << endl;
    return EXIT_FAILURE;
  }           
}

Discussion

The time function returns a time_t type, which is an implementation-defined arithmetic type for representing a time period (a.k.a. a time ...