C++ Check Date is older than 6 month from current date

Question

So I have list of tutor records where it saves date join, and date terminated of them. The program will automatically delete the tutor record once they are terminated after 6 month.

So how do i check the current date is already 6 month or more since the date terminated.

I need to input date join and date terminated.

If you were given a date, how would you work out - on paper - if today's date is more than six months after that date? — Peter
– Peter, Commented Mar 30, 2020 at 8:49
For example the date terminated is 02-09-2019, then I compare today and the date terminated already past 6 months or more. If already past, the program shall delete the tutor records. — Richard Tiozard
– Richard Tiozard, Commented Mar 30, 2020 at 8:54
@RichardTiozard: you don't understand Peter's question: it's not about what you would do once you have determined the date difference. It's about the calculation itself. Let me give you an example: from Jan. 2 to July 2, that's 182 days and from March 2 to Sep. 2, it's 184, so you cannot use the amount of days. You need to compare the years, the months, and the days. How will you do this? — Dominique
– Dominique, Commented Mar 30, 2020 at 9:15
I'm not that sure but I think by using array {31,28,31,30,31,30,31,31,30,31,30,31}. For example if we want to check March 2 to Sep 2, then loop from array[4] value increment until array[10]. and get how many days. Is that okay? — Richard Tiozard
– Richard Tiozard, Commented Mar 30, 2020 at 9:34

Howard Hinnant · Accepted Answer · 2020-03-30 14:24:23Z

Prior to C++20, use a date/time library such as Howard Hinnant's free, open-source C++20 chrono preview library (works back to C++11).

#include "date/tz.h"
#include <chrono>
#include <iostream>
#include <list>

date::year_month_day
current_date()
{
    using namespace date;
    using namespace std::chrono;
    zoned_time zt{current_zone(), system_clock::now()};
    return year_month_day{floor<days>(zt.get_local_time())};
}

struct tutor_record
{
    date::year_month_day joined;
    date::year_month_day terminated;
};

void
trim(std::list<tutor_record>& records)
{
    using namespace date;
    auto limit = current_date() - months{6};
    if (!limit.ok())
        limit = limit.year()/limit.month()/last;
    records.remove_if([limit](auto const& t) {return t.terminated <= limit;});
}

Define a function for getting the current (presumably local) date.
Define a data structure to hold the information.
Get the current date and perform a calendrical computation to subtract 6 months (see this answer for contrasting with chronological computations).
Traverse the list and remove the unwanted items.

This will port to C++20 by:

Remove #include "date/tz.h".
Remove using namespace date;.
Change date:: to std::chrono::.

Remarks:

Works with leap years.
Doesn't require explicit low-level knowledge of how to do calendrical computations.
If the current date in UTC is sufficient, this program can be simplified by the use of the header-only library "date/date.h" which does not require the use of time zones.

return floor<days>(system_clock::now());

Surt · Accepted Answer · 2020-03-30 09:50:05Z

1

c++20 concept code

#include <chrono>

auto then = year_month_day(tutor.join);
auto current = year_month_day(time.now());
auto diff = current - then;
if (diff >= year_month_day(0,6,0))
  delete_tutor(tutor);

answered Mar 30, 2020 at 9:50

Surt

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