setMonth should before setDate: (not safe for Months less than 31 days)

var dt = new Date();

dt.setFullYear(2014);

dt.setMonth(11);

dt.setDate(31);

And setMonth's second parameter also could be used to set date.

var dt = new Date();

dt.setFullYear(2014);

dt.setMonth(11, 31);

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If no arguments are provided for the constructor, it will use the current date and time according to system settings.

So, using setMonth and setDate separately would still cause unexpected result.

If the values set are greater than their logical range, the value will be auto adjusted to the adjacent value.

For example, if today is 2014-09-30, then

var dt = new Date();

dt.setFullYear(2014); /* Sep 30 2014 */

dt.setMonth(1);       /* Mar 02 2014, see, here the auto adjustment occurs! */

dt.setDate(28);       /* Mar 28 2014 */

To avoid this, set the values using the constructor directly.

var dt = new Date(2014, 11, 31);