choose enum

Used with time_zone and zoned_time to indicate how to handle an ambiguous or nonexistent local_time to sys_time conversion.

Syntax

enum class choose { // C++20
    earliest,
    latest
};

Members

Element	Description
earliest	If a time conversion is ambiguous, uses the earlier time point.
latest	If a time conversion is ambiguous, uses the later time point.

Remarks

If the result of the conversion is a nonexistent_local_time, the time in earliest and latest are the same.

Ambiguous conversion error

During the transition from daylight saving time to standard time in the fall, clocks essentially gain an extra hour. This can be confusing because doesn't the transition to standard time mean losing an hour? By falling back an hour, the hour before the transition will be repeated after the clock adjusts for standard time. Consider the change to standard time in New York, which happens on the first Sunday in November at 2:00am. First, 1:00am goes by. At 2am, the clock transitions to standard time, so now it's 1:00am again. That means the time between 1am and 2am will be "repeated", effectively adding an hour.

If a local_time specifies a time during this "extra" hour, it isn't clear how to convert it. Should the converted time be treated as the "first" time that hour happens, or the "second"? If the enum choose isn't specified to indicate which it should be, you'll get an ambiguous_local_time exception.

Nonexistent conversion error

When converting from standard time to daylight saving time, a different problem can arise. During the transition from standard time to daylight saving time in the spring, clocks move ahead an hour. The skipped hour disappears, so even though it appears we've added an hour to the time, 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", resulting in a nonexistent_local_time exception.

Requirements

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

Namespace: std::chrono

Compiler option:/std:c++latest is required.

See also

to_sys
zoned_time constructor