Comparing today's date with another date in moment is returning the wrong date, why?

Based on the documentation (and brief testing), moment.js creates wrappers around date objects. The statement:

var now = moment();

creates a "moment" object that at its heart has a new Date object created as if by new Date(), so hours, minutes and seconds will be set to the current time.

The statement:

var releaseDate = moment("2012-09-25");

creates a moment object that at its heart has a new Date object created as if by new Date(2012, 8, 25) where the hours, minutes and seconds will all be set to zero for the local time zone.

moment.diff returns a value based on a the rounded difference in ms between the two dates. To see the full value, pass true as the third parameter:

 now.diff(releaseDate, 'days', true)
 ------------------------------^

So it will depend on the time of day when the code is run and the local time zone whether now.diff(releaseDate, 'days') is zero or one, even when run on the same local date.

If you want to compare just dates, then use:

var now = moment().startOf('day'); 

which will set the time to 00:00:00 in the local time zone.

RobG's answer is correct for the question, so this answer is just for those searching how to compare dates in momentjs.

I attempted to use startOf('day') like mentioned above:

var compare = moment(dateA).startOf('day') === moment(dateB).startOf('day');

This did not work for me.

I had to use isSame:

var compare = moment(dateA).isSame(dateB, 'day');