Question

I’m struggling a little getting some C++ code to cooperate. Is there a straight forward way to subtract two dates that are strings? I’m trying to get the number of days between them. They are strings, YYYYMMDD. Is the Boost library the right direction to go?

Answer 1

Yes, Boost Date_Time can certainly do that even others may suggest “lighter” alternatives.

Here is the example about the days alive which does almost exactly your problem (just add a second date parsing):

/* Short example that calculates the number of days since user was born.
 * Demonstrates comparisons of durations, use of the day_clock,
 * and parsing a date from a string.
 */

#include "boost/date_time/gregorian/gregorian.hpp"
#include <iostream>

int
main() 
{

  using namespace boost::gregorian;
  std::string s;
  std::cout << "Enter birth day YYYY-MM-DD (eg: 2002-02-01): ";
  std::cin >> s;
  try {
    date birthday(from_simple_string(s));
    date today = day_clock::local_day();
    days days_alive = today - birthday;
    days one_day(1);
    if (days_alive == one_day) {
      std::cout << "Born yesterday, very funny" << std::endl;
    }
    else if (days_alive < days(0)) {
      std::cout << "Not born yet, hmm: " << days_alive.days() 
                << " days" <<std::endl;
    }
    else {
      std::cout << "Days alive: " << days_alive.days() << std::endl;
    }

  }
  catch(...) {
    std::cout << "Bad date entered: " << s << std::endl;
  }
  return 0;
}