This exception is thrown when attempting to convert a local_time to a non-existent sys_time.

class nonexistent_local_time : public runtime_error; // C++20

During the transition from standard time to daylight saving time in the spring, clocks essentially lose an hour. This can be confusing because doesn't the transition to daylight saving time mean adding an hour? By "springing forward" an hour, the hour following the transition is effectively removed. Consider the change to daylight saving time in New York, which happens on the second Sunday in March at 2am. At 2am, the clock transitions to daylight savings time and now reads 3:00am. If the local_time being converted is 2:30am, for example, that time is during the period that was "removed" and so is "nonexistent".

The following example demonstrates a nonexistent time conversion error. In this example, daylight saving time in New York starts at 2:00am. The time being converted is 2:30am. That's during the hour that is removed because of the transition from standard time to daylight saving time. So, a nonexistent_local_time exception is thrown.

#include <chrono>
#include <iostream>

using namespace std::chrono;
    
int main()
{
    try
    {
        // The following will throw an exception because the local time being converted is during
        // the hour that is "removed" when the clock advances an hour for daylight saving time.
        auto zt = zoned_time{"America/New_York", local_days{Sunday[2]/March/2016} + 2h + 30min};
    } catch (const nonexistent_local_time& e)
    {
        std::cout << e.what() << '\n';
    }
    return 0;
}

2016-03-13 02:30:00 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

Header: <chrono> (since C++20)

Namespace: std::chrono

Compiler Option: /std:c++latest

Constructs a nonexistent_local_time.

template<class Duration>
nonexistent_local_time(const local_time<Duration>& tp, const local_info& i);

tp
The local_time that failed conversion.

i
Information about the attempted conversion. See local_info for details.

You typically won't create this exception. It's thrown by functions that convert a local_time to a sys_time.

Gets a string describing why the time is non-existent.

[[nodiscard]] virtual const char* what() const noexcept;

A string describing why the time is non-existent. For example:

2016-03-13 02:30:00 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

<chrono>
to_sys
ambiguous_local_time
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