Question

I’m working with Javascript dates, and I’m getting a bit confused with trying to take a date from a string.

This is the code I have:

var formatDate = function(dateObj) {
    // make sure date values are two digits and months start at 1
    var adjMonth = dateObj.getMonth() + 1;
    var adjDate = dateObj.getDate();
    if (adjMonth < 10) adjMonth = '0' + adjMonth;
    if (adjDate < 10) adjDate = '0' + adjDate;

    // build and return dateStr
    var dateStr = dateObj.getFullYear() + '-' + adjMonth + '-' + adjDate;
    return dateStr;
};

$(document).ready(function() {
    var testIn1 = "2012-02-01";
    var testDate1 = new Date(testIn1);
    var testDate1Str = formatDate(testDate1);

    var testIn2 = "2012-01-31";
    var testDate2 = new Date(testIn2);
    var testDate2Str = formatDate(testDate2);

    $('#output').html("---Input = '" + testIn1 + "':<br>" + testDate1 + "<br>" + testDate1Str + "<br>"
                     +"---Input = '" + testIn2 + "':<br>" + testDate2 + "<br>" + testDate2Str + "<br>");
});​

Results I get from this are:

---Input = '2012-02-01':
Tue Jan 31 2012 18:00:00 GMT-0600 (CST)
2012-01-31
---Input = '2012-01-31':
Mon Jan 30 2012 18:00:00 GMT-0600 (CST)
2012-01-30

Which makes no sense to me, why are the days one off? Doesn’t seem sensical to get 2012-01-31 from 2012-02-01… What am I missing here?

Answer 1

It looks like Date.parse uses 00:00:00 GMT if you don’t pass a time, and it will be 18:00:00 the previous day in your time zone (GMT-6). If you do pass an explicit time, then this behaviour is suppressed:

Date.parse(testIn1 + " 00:00:00");