cpp/chrono/choose

The scoped enumeration can be passed to certain member functions of std and std to control how ambiguous or nonexistent local times should be resolved. Passing causes the earlier time point to be returned, while passing  causes the later time point to be returned. (For nonexistent local times, these two time points are identical.)

If a is not passed and an ambiguous or nonexistent local time is encountered, a std or std exception (as applicable) will be thrown.