Question

I want to pick up date from screen, add 1 day and put back to screen.

<input type='text'value='20101231' id='from_date'>

<script>    

    function date_move(direction){              
        var from_date = document.getElementById('from_date').value;                 
        var YYYY = from_date.substring(0,4);
        var MM = from_date.substring(4,6);
        var DD = from_date.substring(6,8);      
        var jsCurDate = new Date( parseInt(YYYY,10), parseInt(MM,10)-1, parseInt(DD,10) );
        var newDate = addOrSubtracDays(jsCurDate,direction)             
        document.getElementById('from_date').value = newDate;   
    }


    function addOrSubtracDays(jsCurDate,daysOffset){        
        var daylength= 1*24*60*60*1000;
        var newDate = new Date(jsCurDate + daylength);
        alert(newDate)      
    }


date_move(+1);

</script>   

Answer 1

You can’t use element.value to get the contents of an arbitrary HTML element, .value only works on form elements. Use element.innerHTML instead:

var from_date = document.getElementById('from_date').innerHTML;

Also, your use of substring is wrong. I think substr is more clear:

var YYYY = from_date.substr(0,4);
var MM = from_date.substr(4,2);
var DD = from_date.substr(6,2);

In addOrSubtracDays (shouldn’t that be addOrSubtractDays?) you want the number of milliseconds, so you should call Date.getTime. Moreover, you actually want to return the new value:

return new Date(jsCurDate.getTime() + daylength * daysOffset);

When changing #from_date you probably want your date to be in the same format. For this, I extend Date‘s prototype with a new method (and String with a helper function):

String.prototype.padLeft = function (length, character) {
    return new Array(length - this.length + 1).join(character || ' ') + this;
};

Date.prototype.toFormattedString = function () {
    return [String(this.getFullYear()),
            String(this.getMonth()+1).padLeft(2, '0'),
            String(this.getDate()).padLeft(2, '0')].join('');
};

After this, we can simply call toFormattedString:

document.getElementById('from_date').innerHTML = newDate.toFormattedString();

BTW, you don’t need to explicitly convert YYYY, MM and DD to integers with base 10; those are converted automatically (without assuming a string beginning with 0 is an octal number):

var jsCurDate = new Date(YYYY, MM - 1, DD);

For the sake of clarity, the complete script now reads:

String.prototype.padLeft = function (length, character) {
    return new Array(length - this.length + 1).join(character || ' ') + this;
};

Date.prototype.toFormattedString = function () {
    return [String(this.getFullYear()),
            String(this.getMonth()+1).padLeft(2, '0'),
            String(this.getDate()).padLeft(2, '0')].join('');
};

function date_move(direction){
    var from_date = document.getElementById('from_date').innerHTML;
    var YYYY = from_date.substr(0,4);
    var MM = from_date.substr(4,2);
    var DD = from_date.substr(6,2);
    var jsCurDate = new Date(YYYY, MM - 1, DD);
    var newDate = addOrSubtracDays(jsCurDate, direction);
    document.getElementById('from_date').innerHTML = newDate.toFormattedString();
}

function addOrSubtracDays(jsCurDate, daysOffset){
    var daylength= 1*24*60*60*1000;
    return new Date(jsCurDate.getTime() + daylength * daysOffset);
}

date_move(+1);