Excel Date Library

datelib

/**
 * Returns the day of the week for the provided date,
 * where Monday is the first day.
 */
dayOfWeek = LAMBDA(datetime, WEEKDAY(datetime, 2));

/**
 * Returns the name for the day of the week for the provided date.
 */
dayOfWeekName = LAMBDA(datetime, TEXT(datetime, "dddd"));

dayOfYear = LAMBDA(datetime, datetime - DATE(YEAR(datetime), 1, 1) + 1);

daysInMonth = LAMBDA(datetime, DAY(EOMONTH(datetime, 0)));

daysInYear = LAMBDA(datetime, DATE(YEAR(datetime) + 1, MONTH(datetime), DAY(datetime)) - datetime);

endOfQuarter = LAMBDA(datetime, EOMONTH(DATE(YEAR(datetime), ROUNDDOWN(MONTH(datetime) / 3, 0) * 3, 1), 0));

endOfWeek = LAMBDA(datetime, datetime + 7 - WEEKDAY(datetime));

endOfYear = LAMBDA(datetime, DATE(YEAR(datetime), 12, 31));

quarterOfYear = LAMBDA(datetime, ROUNDDOWN(MONTH(datetime) / 3, 0) + 1);

startOfMonth = LAMBDA(datetime, DATE(YEAR(datetime), MONTH(datetime), 1));

startOfQuarter = LAMBDA(datetime, DATE(YEAR(datetime), ROUNDDOWN(MONTH(datetime) / 3, 0) * 3 + 1, 1));

startOfWeek = LAMBDA(datetime, datetime - WEEKDAY(datetime) + 1);

startOfYear = LAMBDA(datetime, DATE(YEAR(datetime), 1, 1));

weekOfMonth = LAMBDA(datetime, ROUNDUP(DAY(datetime + 7 - WEEKDAY(datetime)) / 7, 0));

weekOfYear = LAMBDA(datetime, WEEKNUM(datetime));

// Hidden test function
_do_test = LAMBDA(t,
    HSTACK(
        VSTACK(
            "dayOfWeek",
            "dayOfWeekName",
            "dayOfYear",
            "daysInMonth",
            "daysInYear",
            "endOfQuarter",
            "endOfWeek",
            "endOfYear",
            "quarterOfYear",
            "startOfMonth",
            "startOfMonth",
            "startOfQuarter",
            "startOfWeek",
            "startOfYear",
            "weekOfMonth",
            "weekOfYear"
        ),
        VSTACK(
            dayOfWeek(t),
            dayOfWeekName(t),
            dayOfYear(t),
            daysInMonth(t),
            daysInYear(t),
            endOfQuarter(t),
            endOfWeek(t),
            endOfYear(t),
            quarterOfYear(t),
            startOfMonth(t),
            startOfMonth(t),
            startOfQuarter(t),
            startOfWeek(t),
            startOfYear(t),
            weekOfMonth(t),
            weekOfYear(t)
        )
    )
);