Methods getIndexOf*PeriodOnWhichDateIsIncluded - lapaliv/datetime-formatter GitHub Wiki

Your friend works 3 days, then he rests 2 days, then he works 3 days and so on. He started working on May 12, 2012. You need to find out if he will work on January 1, 2020.

const formatter = new DateTimeFormatter(2020, 1, 1);
const startedAt = new DateTimeFormatter(2012, 5, 12);

const indexOfPeriod = formatter.getIndexOfDayPeriodOnWhichDateIsIncluded(startedAt, [3, 2]);
// or
const indexOfPeriod = formatter.getIndexOfDayPeriodOnWhichDateIsIncluded(startedAt, 3, 2);

If the indexOfPeriod is 0, then this means that January 1, 2020 will fall on the first period (3 days). If the indexOfPeriod is equal to 1 - for the second period (2 days).

You can specify an unlimited number of periods