gistfile1.txt

class DenseDates {
    private DenseDates() {}

    // 0 == Monday, 6 == Sunday
    private static int epochDayToDayOfWeek0Based(long epochDay) {
        return (int)Math.floorMod(epochDay + 3, 7);
    }

    public static int daysBetween(long fromEpochDay, long toEpochDay) {
        // http://stackoverflow.com/questions/1617049/calculate-the-number-of-business-days-between-two-dates
        final int fromDOW = epochDayToDayOfWeek0Based(fromEpochDay);
        final int toDOW = epochDayToDayOfWeek0Based(toEpochDay);
        long calcBusinessDays = ((toEpochDay - fromEpochDay) * 5 + (toDOW - fromDOW) * 2) / 7;

        if (toDOW   == 6) calcBusinessDays -= 1;
        if (fromDOW == 6) calcBusinessDays += 1;
        return (int)calcBusinessDays;
    }

    public static long addDays(long epochDay, int n) {
        // https://alecpojidaev.wordpress.com/2009/10/29/work-days-calculation-with-c/
        // NB: in .NET, Sunday == 0, but in our code Monday == 0
        final int dow = (epochDayToDayOfWeek0Based(epochDay) + 1) % 7;
        final int wds = n + (dow == 0 ? 1 : dow); // Adjusted number of working days to add, given that we now start from the immediately preceding Sunday
        final int wends = n < 0 ? ((wds - 5) / 5) * 2
                                : (wds / 5) * 2 - (wds % 5 == 0 ? 2 : 0);
        return epochDay - dow + // Find the immediately preceding Sunday
               wds +            // Add computed working days
               wends;           // Add weekends that occur within each complete working week
    }
}