fd::convert_weekday — Convert an onset's 0-based weekday number from one reference system to another.
// In header: <fd/interval_types.h> constexpr int convert_weekday(const interval_onset & onset, iso_weekday_value outputFirstDow);
Parameters:
outputFirstDow
First day of week according to ISO 8601.
Returns:
Converted weekday number modulo 7.