Javascript creates dates that dont exist

Question

I wrote some JS to try and come up with with the last date of the month a user selects, I am trying to figure out why it is coming up with dates that dont exist, such as 6/31/2015, the problem is probably around how I am calculating end of month, but I just cant see my way around to the right conclusion here. Its an interesting bug so I thought I'd see if you guys have a solution.

var ealiest_effective_date = new Date(Date.parse("#{ruby_earliest_time_which_is_confirmed_correct.try(:strftime, "%m/%d/%Y")}"));
var minimumtime = Date.parse(new Date(ealiest_effective_date.getFullYear(), ealiest_effective_date.getMonth() + 2, 0));

$('.selected-date').change(function (e) {
    date = Date.parse($(this).val());
    user_selected_date = new Date(date);

    console.log(user_selected_date.getMonth()+'/'+user_selected_date.getDate()+'/'+user_selected_date.getFullYear());

    var possibleResultantEndOfMonth = Date.parse(new Date(user_selected_date.getFullYear(), user_selected_date.getMonth() + 2, 0));
    var terminationEndOfMonth = new Date(Math.max(possibleResultantEndOfMonth,minimumtime));
    var end_of_month_date = terminationEndOfMonth.getMonth()+'/'+terminationEndOfMonth.getDate()+'/'+terminationEndOfMonth.getFullYear();

    $('#should_be_end_of_month').val(effective_date);
 });

score 2 · Accepted Answer · answered Jul 31 '15 at 01:50

2

.getMonth() JavaScript method returns month as zero based value. E.g. January is 0 and December is 11. In your example 6/31/2015 should actually be July 31, not June

Also I would be careful with the following code

date = Date.parse($(this).val());

It may not work for people with different regional settings.

answered Jul 31 '15 at 01:50

Maksym Kozlenko

10,273
2
66
55

Winner winner! Why are dates 0 based?! – Jay Jul 31 '15 at 02:41
http://stackoverflow.com/questions/15799514/why-does-javascript-getmonth-count-from-0-and-getdate-count-from-1 – Maksym Kozlenko Jul 31 '15 at 05:13

score 1 · Answer 2 · answered Jul 31 '15 at 00:11

1

I dont know your logic or calculation but when working with dates in JavaScript I highly recommend to use Moment.js which allows to do dates manipulation in an easy way and also allows to handle timezones. Take a look to it, I think it could help you.

d = moment("20150231", "YYYYMMDD")
d.isValid()
false
moment("20150228", "YYYYMMDD").isValid()
true

answered Jul 31 '15 at 00:11

Martin Alderete

736
5
9

Agreed. Best library so far fro working with dates and localization – Maksym Kozlenko Jul 31 '15 at 01:56

score 1 · Answer 3 · answered Jul 31 '15 at 00:12

1

Unless you have declared it earlier, your variable is:

var ealiest_effective_time

But you're referencing:

ealiest_effective_date

answered Jul 31 '15 at 00:12

Charlie Harster

83
6

Just me changing vars for the web. not the issue, but upvote anyway! – Jay Jul 31 '15 at 01:43

Javascript creates dates that dont exist

3 Answers3