I am currently writing some sort of javascript based client calendar and observed some issues. All over the net, I can find code samples, where people use day overflows in the Date constructor.

i.e.

// get the first day of the next month
var myDate = new Date(someDate.getFullYear(),someDate.getMonth(),32);
myDate.setDate(1);

The general idea of this concept is, that since there is no month with 32 days, the constructor will create a date within the next month. I saw even codesample with negative overflows:

i.e.

// get the last day of the previous month
var myDate = new Date(someDate.getFullYear(),someDate.getMonth(),1);
myDate.setDate(-1);

Now while this seems to work in many cases, I finally found a contradiction:

// this prints "2012-12-30" expected was "2012-12-31"
var myDate = new Date(2013,0,1);
myDate.setDate(-1);

Further examination finally revealed that dates like new Date(2013,0,23) or new Date(2013,0,16) bined with setDate(-1) all end up in "2012-12-31". Finally I observed that using -1 seems to subtract two days (for getting the expected result setDate(0) has to be used).

Is this a bug in the browser implementations or are the code samples spread accross the internet crap??

Furthermore, is this setDate with positive and negative overflow secure to be used and uniformly implemented by all major browsers?

I am currently writing some sort of javascript based client calendar and observed some issues. All over the net, I can find code samples, where people use day overflows in the Date constructor.

i.e.

// get the first day of the next month
var myDate = new Date(someDate.getFullYear(),someDate.getMonth(),32);
myDate.setDate(1);

The general idea of this concept is, that since there is no month with 32 days, the constructor will create a date within the next month. I saw even codesample with negative overflows:

i.e.

// get the last day of the previous month
var myDate = new Date(someDate.getFullYear(),someDate.getMonth(),1);
myDate.setDate(-1);

Now while this seems to work in many cases, I finally found a contradiction:

// this prints "2012-12-30" expected was "2012-12-31"
var myDate = new Date(2013,0,1);
myDate.setDate(-1);

Further examination finally revealed that dates like new Date(2013,0,23) or new Date(2013,0,16) bined with setDate(-1) all end up in "2012-12-31". Finally I observed that using -1 seems to subtract two days (for getting the expected result setDate(0) has to be used).

Is this a bug in the browser implementations or are the code samples spread accross the internet crap??

Furthermore, is this setDate with positive and negative overflow secure to be used and uniformly implemented by all major browsers?

• javascript

• 1 As reference, the specification: es5.github./#x15.9.5.36. – Felix Kling Commented Jan 2, 2013 at 11:48

2 Answers 2

From MDN:

If the parameter you specify is outside of the expected range, setDate attempts to update the date information in the Date object accordingly. For example, if you use 0 for dayValue, the date will be set to the last day of the previous month.

It's logical if you think about it: setDate(1) sets the date to the first of the month. To get the last day of the previous month, that is, the day before the first of this month, you subtract one from the argument and get 0. If you subtract two days (1 - 2) you get the second to last day (-1).

are [..] code samples spread accross the internet crap?

Yes. This is true at least 90% of the time.

At MDN they say:

If the parameter you specify is outside of the expected range, setDate attempts to update the date information in the Date object accordingly. For example, if you use 0 for dayValue, the date will be set to the last day of the previous month.

So you're getting coherent results:

 1 - Jan 1
 0 - Dec 31
-1 - Dec 30
-2 - Dec 29

Edit: It may look counter-intuitive if you think of it as a mere relative value, such as PHP's strtotime() function:

strtotime('-1 day');

It's not the case ;-)