Is Javascript calculating day differences wrong? That's because you also need to add local time settings to you calculation.

This function does exactly what you want.

function days_between(date1, date2) {
var ONE_DAY = 1000 * 60 * 60 * 24;
var timeZoneOffset = new Date().getTimezoneOffset()
var date1_ms = date1.getTime() - timeZoneOffset*60*1000;
var date2_ms = date2.getTime() - timeZoneOffset*60*1000;

var difference = parseInt( date1_ms / ONE_DAY) - parseInt( date2_ms / ONE_DAY );
return difference;
}

And you use like this.
alert( days_between( new Date("2010/02/05 12:00"), new Date("2010/02/04 23:00") ));

Be happy and do less.