Javascript's "new Date()" returns different dates for '2011-02-1' and '2011-02-01'

Question

Here's my experiment:

> new Date('2011-02-1')
2011-02-01T08:00:00.000Z
> new Date('2011-02-01')
2011-02-01T00:00:00.000Z

Does anybody know why there are 8 hours difference between the two?

Thanks!

Answer 1

This is actually up to the browser or JavaScript runtime implementation to determine what happens. The Date constructor conforms to the format YYYY-MM-DDTHH:mm:ss.sssZ which is a simplification of ISO 8601 Extended Format. If parsing that fails, it is implementation-specific what happens.

Essentially, since you failed to specify DD for 2011-02-1, it's up to the browser to call the shots on what you'll get. For example, in Google Chrome and Firefox I get 2011-02-01T00:00:00.000Z yet in Safari I just get an error, Invalid Date.

See the ECMAScript 5 specification:

[…] the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (15.9.1.15) and that could not be produced in that implementation by the toString or toUTCString method.

Note that the Date constructor uses the same parsing algorithm as Date.parse. This is why it's highly discouraged to use a string for the constructor. Use the multi-argument version that is standardized. See MDN.