Simple javascript date math... not really

Question

I am trying to create a simple script that gives me the next recycling date based on a biweekly schedule starting on Wed Jul 6, 2011. So I've created this simple function...

    function getNextDate(startDate) {
        if (today <= startDate) {
            return startDate;
        }
        // calculate the day since the start date.
        var totalDays = Math.ceil((today.getTime()-startDate.getTime())/(one_day));
        // check to see if this day falls on a recycle day 
        var bumpDays = totalDays%14;  // mod 14 -- pickup up every 14 days...
        // pickup is today
        if (bumpDays == 0) {
            return today;
        }
        // return the closest day which is in 14 days, less the # of days since the last
        // pick up..
        var ms =  today.getTime() + ((14- bumpDays) * one_day);
        return new Date(ms);
    }

and can call it like...

 var today=new Date();
 var one_day=1000*60*60*24;  // one day in milliseconds
 var nextDate = getNextDate(new Date(2011,06,06));

so far so good... but when I project "today" to 10/27/2011, I get Tuesday 11/8/2011 as the next date instead of Wednesday 11/9/2011... In fact every day from now thru 10/26/2011 projects the correct pick-up... and every date from 10/27/2011 thru 2/28/2012 projects the Tuesday and not the Wednesday. And then every date from 2/29/2012 (leap year) thru 10/24/2012 (hmmm October again) projects the Wednesday correctly. What am I missing? Any help would be greatly appreciated..

V

Answer 1

The easiest way to do this is update the Date object using setDate. As the comments for this answer indicate this isn't officially part of the spec, but it is supported on all major browsers.

You should NEVER update a different Date object than the one you did the original getDate call on.

Sample implementation:

var incrementDate = function (date, amount) {
    var tmpDate = new Date(date);
    tmpDate.setDate(tmpDate.getDate() + amount)
    return tmpDate;
};

If you're trying to increment a date, please use this function. It will accept both positive and negative values. It also guarantees that the used date objects isn't changed. This should prevent any error which can occur if you don't expect the update to change the value of the object.

---

Incorrect usage:

var startDate = new Date('2013-11-01T11:00:00');
var a = new Date();
a.setDate(startDate.getDate() + 14)

This will update the "date" value for startDate with 14 days based on the value of a. Because the value of a is not the same is the previously defined startDate it's possible to get a wrong value.