Determine if a week is odd or even

Question

I have debugged this legacy code, and would like a sanity check on it.

The purpose of it is to allow someone to choose a delivery frequency for shipping a product. If someone wants their product Every Other Week, the system needs to determine if they should get an order next week, or two weeks from now. We call it A week, or B Week.

Keep in mind I did not write this, I am just trying to make sense of it and would like some help evaluating its accuracy:

if (date("l") == "Monday" ) {
            $start = 0;
        } else if (date("l") == "Tuesday" || date("l") == "Wednesday" || date("l") == "Thursday" || date("l") == "Friday" || date("l") == "Saturday"|| date("l") == "Sunday") {
        $start = -1;
    }

    // if changing to every other week set to next week's a/b-ness
    $a_week_tid = 34;
    $b_week_tid = 35;

    $every_other_week_frequency_id = 32;

    if ($delivery_frequency == $every_other_week_frequency_id) {
        $julian = (int) (strtotime('Monday +' . $start . ' week') / 86400);
        $julian_week = ($julian-4) / 7;
        if ($julian_week % 2) {
            $today_a_or_b = $b_week_tid;
            $next_week_a_or_b = $a_week_tid;
            $a_or_b_week_string = '(A Week)';
        } else {
            $today_a_or_b = $a_week_tid;
            $next_week_a_or_b = $b_week_tid;
            $a_or_b_week_string = '(B Week)';
        }
    } else {
        $next_week_a_or_b = NULL;
        $a_or_b_week_string = NULL;
    }

This code is not commented or documented. The part that confuses me is:

1. Why is 4 subtracted from Julian, then divided by 7?
2. If today is Monday, $julian_week is 2129, and 2129 % 2 evaluates TRUE. Is that correct?
3. Is this even how it should be done? Can't I rewrite this using date('w') a lot easier?

Answer 1

Yeah using date would totally be easier, plus it takes into account leap years, daylight saving time, all that extra stuff you don't want to have to deal with.

if (date('W')%2==1)

That's SOOOO much easier to maintain than the above.

Answer 2

I don't believe you can use date("W") in this case. According to the ISO calculation, on occasion, there will be years with 53 weeks. In those years, Week 53 is followed by Week 01, both odd numbers, and an A/B calculation based on Even/Odd ISO week number would result in two successive A or B weeks.

The original calculation determines the number of days from the UNIX epoch of the present Monday, or of the most recent Monday if today is not a Monday. The -4 causes the A/B week labels to change on Thursdays. Even/oddness of a week is determined from a fixed date (the Unix Epoch), so there will be no discontinuity in the oscillation of A/B-ness using the original code.