Is there a way to determine if a date/time does not exist?

Question

Fun fact that I'm sure most of us who get to play in the time realms know - there are date/times that can appear valid but actually do not exist, e.g. 2:30 AM on a daylight savings switching time.

Is there a way in C++ (standard or Windows) to figure out if a given date/time is valid in a given time zone specification?

Answer 1

Using this free, open-source library:

#include "tz.h"
#include <iostream>

int
main()
{
    using namespace date;
    using namespace std::chrono_literals;
    try
    {
        auto zone = locate_zone("America/New_York");
        zone->to_sys(local_days{mar/13/2016} + 2h + 30min);
    }
    catch (const std::exception& e)
    {
        std::cout << e.what() << '\n';
    }
}

The output of this program is:

2016-03-13 02:30 is in a gap between
2016-03-13 02:00:00 EST and
2016-03-13 03:00:00 EDT which are both equivalent to
2016-03-13 07:00:00 UTC

In short, this program attempts to translate a locale date/time of 2016-03-13 02:30:00 to UTC using the timezone "America/New_York". The translation throws an exception because the local time doesn't exist. An exception is also thrown (with a different error message), if the local time is ambiguous, such as when setting the local clock back from daylight saving.

The library also provides syntax for avoiding these exceptions if that is the desire.

This works on VS-2013, VS-2015, clang, and gcc. It requires C++11, C++14 or C++1z.

The link above points to the documentation. Here is the github repository:

https://github.com/HowardHinnant/date

Answer 2

Using Windows-specific functions, you can make a call to TzSpecificLocalTimeToSystemTime() followed by a call to SystemTimeToTzSpecificLocalTime() with the relevant time zone.

Upon completion, if the two differ you have an invalid time as TzSpecificLocalTimeToSystemTime converts the invalid time to a "real" time.

bool IsValidTime(TIME_ZONE_INFORMATION tz, SYSTEMTIME st)
{
    SYSTEMTIME utcSystemTime, st2;
    TzSpecificLocalTimeToSystemTime(&tz, &st, &utcSystemTime);
    SystemTimeToTzSpecificLocalTime(&tz, &utcSystemTime, &st2);

    return (st != st2);
}

Answer 3

The time_t format is an integer value (seconds since 1/1/1970 00:00 UTC) so every possible value of time_t exists (within the limits of the integer type used of course).

On the other hand, not all possible values of "struct tm" (where you have members for day, month, year, hour, minute and second) exist (and this is probably what you mean). To check whether a "struct tm" exists, you have to do the following:

• Set the member tm_isdst to -1. This indicates that you don't know whether its DST or not.
  • Convert the "struct tm" to a time_t using mktime().
  • Convert the time_t back to "struct tm" using localtime().
  • Compare the resulting "struct tm" with the initial "struct tm", ignore the tm_isdst field (or use it to see whether you are in DST or not).