Question

In my JavaScript code I have to variables (x and y) with datetime values. All of the following statements return false:

• x < y -> false
• x > y -> false
• x == y -> false
• x === y -> false

When I debug the code both x and y have the same value ‘Sat Jan 1 12:00:00 UTC+0100 2011’ and the same type.

I am using IE9.

Is there a different way to check if two datetime values are equal?

update (code) – I tried to extract some relevant parts, whole project is very big:

var data;


//Dates are loaded from a webservice and returned as strings; they are parsed with the following function


data = parseD(getDataFromService());

function parseD (data) {
    for (var i = 0; i < data.length; i++) {
        var d = data[i];
        for (var key in d) {
            if (d[key] && d[key].toString().substring(0, 6) === "/Date\(")
                d[key] = new Date(parseInt(d[key].toString().substr(6)));
        }
    }
    return data;
};

function getCssForCell(row, columnID, dataContext, value) {
            var css = "";
            if (columnID === "VF" && value) {
                if (row > 0) {
                    if (data[row].VF < addDays(data[row - 1].VF, 2)) {
                        css = "error_cell";
                    }
                }
            }
            if (columnID === "VU" || columnID === "VF") {
                if (dataContext.VF && dataContext.VU) {
                    if (dataContext.VU <= dataContext.VF) {
                        css = "error_cell";
                    }
                }
            }
            if (columnID === "Rate" && row < (strict_mode ? data.length - 1 : data.length - 2)) { //value for Rate must be assigned
                if (value === undefined || value === null) {
                    css = "error_cell";
                }
            }
            if (columnID === "VF" && row < (strict_mode ? data.length - 1 : data.length - 2)) {
                if (value === undefined || value === null) {
                    css = "error_cell";
                }
            }

            if (columnID === "Rate" && value < 0) {
                css = "error_cell";
            }

            //PROBLEM IN NEXT LINE
            if (columnID === "VU" && row < data.length - 1 && dataContext.VU && data[row + 1] && addDays(dataContext.VU, 1) != data[row + 1].VF ) {
                css = "error_cell";
            }

            return css;
        }

function addDays(date, days) {
    if (date) {
        var result = new Date(date.getFullYear(), date.getMonth(), date.getDate(), date.getHours());
        result.setDate(result.getDate() + days);
        return result;
    }
    return date;
}

Answer 1

You should not compare Date object instances, but their values, compare using .getTime() method instead:

x.getTime() < y.getTime();
x.getTime() > y.getTime();
x.getTime() == y.getTime();
x.getTime() === y.getTime();

.getTime() returns the numeric value corresponding to the time for the specified date according to universal time, this value is the number of milliseconds since 1 January 1970 00:00:00 UTC, also it is equivalent to .valueOf() method.