Subtracting days/months/years from a Date object

Question

var inputDate     = '20/4/2010'.split('/');
var dateFormatted = new Date(parseInt(inputDate[2]), parseInt(inputDate[1]), parseInt(inputDate[0]));

var expiryDate = (dateFormatted.getDate() - 1) + '/' + dateFormatted.getMonth() + '/' + (dateFormatted.getFullYear() + year);

This is the Javascript code I'm using to work out an expiry date given a user inputted date. Currently, the expiry date is original date minus one day and original year minus X.

The problems with this code, firstly, it doesn't take into account invalid dates. For example, if the user supplied date is '1/10/2010', the expiry date will be '0/10/2013' (assuming the expiry date is +3 years).

I could do something like:

var inputDate = '20/4/2010'.split('/');
var day       = parseInt(inputDate[0]);
var month     = parseInt(inputDate[1]);
var year      = parseInt(inputDate[2]);

if (day < 1)
{
    if (month == ...)
    {
        day   = 31
        month = month - 1;
    }
    else
    {
        day   = 30
        month = month - 1;
    }
}

var dateFormatted = new Date(parseInt(inputDate[2]), parseInt(inputDate[1]), parseInt(inputDate[0]));
var expiryDate    = (dateFormatted.getDate() - 1) + '/' + dateFormatted.getMonth() + '/' + (dateFormatted.getFullYear() + year);

But more problems arise... Firstly, the code gets a little convoluted. Secondly, this check would have to be done on the day. and then the month. Is there a cleaner, simpler way?

Also, there's a certain circumstance that would involve me needing to calculate the expiry date to the 'end of the month' for that date. For example:

Expiry date is: +3 years

User date is: '14/10/2010'
Expiry date is: '31/10/2013'

I was hoping the Date object would support these calculations but according to https://developer.mozilla.org/en/JavaScript/Reference/global_objects/date, it seems not...

Answer 1

Easy way to see if a date inputed is a valid date:

var d = Date.parse('4/20/2010');
if (isNaN(d.valueOf())) {
 alert ("bad date value"); 
}

Then, here is a dateAdd function that I use regularly. Extends the Date object, so it's easy to use:

Date.prototype.dateAdd = function(size,value) {
    value = parseInt(value);
    var incr = 0;
    switch (size) {
        case 'day':
            incr = value * 24;
            this.dateAdd('hour',incr);
            break;
        case 'hour':
            incr = value * 60;
            this.dateAdd('minute',incr);
            break;
        case 'week':
            incr = value * 7;
            this.dateAdd('day',incr);
            break;
        case 'minute':
            incr = value * 60;
            this.dateAdd('second',incr);
            break;
        case 'second':
            incr = value * 1000;
            this.dateAdd('millisecond',incr);
            break;
        case 'month':
            value = value + this.getUTCMonth();
            if (value/12>0) {
                this.dateAdd('year',value/12);
                value = value % 12;
            }
            this.setUTCMonth(value);
            break;
        case 'millisecond':
            this.setTime(this.getTime() + value);
            break;
        case 'year':
            this.setFullYear(this.getUTCFullYear()+value);
            break;
        default:
            throw new Error('Invalid date increment passed');
            break;
    }
}

Then just use:

 var d = new Date();
 d.dateAdd('day', -1).dateAdd('year', 3);

T'da

Answer 2

A similar question has been answered here:

How to add/subtract dates with javascript?

Similar thing can be done for months and years.

For e.g.

     var date = new Date('2011','01','02');
     alert('the original date is '+date);
     var newdate = new Date(date);
     newdate.setMonth(newdate.getMonth() - 7);
     var nd = new Date(newdate);
     alert('the new date is '+nd);