September has 31 days when it should have 30

[brenoguim]

I did not check other months.

[thamara]

Probably doing something wrong with the month number as js has the base habit of describing months from 0 to 11, and not 1 to 12

[dsrariel]

All days are right-shifted one month (e. g., March has 28 days on 2019). I search through _getBody() method on js/calendar.js. Lines 489-450 have the following code:

var d = new Date(this.year, this.month, 0),
            monthLength = d.getDate();

In that case, monthLength has the number of days of this.month - 1, that's why everything is shifted (just adding a one will cause other things to crash, though 😅 ).

[tupaschoal]

Apparently:

When you pass a 0 to the third argument instead, it uses the last day of the previous month

So calls calculating monthLength should take this into consideration.