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kotlinx.datetime.DayOfWeek is ordered according to ISO-8601, where Monday is 1 and Sunday is 7. java.util.Calendar's day of week constants are ordered with Sunday is 1 and Saturday is 7. This function converts between the two. It is called toJewishDayOfWeek because the Torah describes Saturday as the 7th day. Note: The caller should prefer to compare DayOfWeeks directly, instead of converting to day numbers. This is to improve readability and possible future-proofing.