Javascript need two date instances Keep getting references instead

Question

How to have a date and subtract 1 year and have a second date for use in display and caculations?

I'm a newbe with JS and find myself fighting the date object
I have declared both currentDate and purchaseDate As var new date() in the globals area But now in a function I try to assign the Original date as "currentDate and the purchaseDate" as one year later,, The alert shows that I have changed the vaue of currentDate rather than just the value of purchaseDate as I intended..

Not what I want! So I get pass by reference vs by value but don't know how to get this so that I have two separate values for the currentDate and the purchaseDate (one year earlier)

currentDate = data.getValue(0,2);
purchaseDate = valueOf(data.getValue(0,2));
purchaseDate.setFullYear(purchaseDate.getFullYear()-1);

alert(purchaseDate);

so this code fails also; That is,, purchase date is 1 year back but so is current date

currentDate = data.getValue(0,2);
    purchaseDate = data.getValue(0,2);
            purchaseDate.setFullYear(purchaseDate.getFullYear()-1);
alert(purchaseDate);

Answer 1

The code which you posted is too ambiguous to reliably point the root cause of your problem (it's unclear what valueOf() is doing), but basically, you indeed need to create a new Date instance based on the time of the other Date. Here's how you could do this, assuming that currentDate is a real Date as well.

var purchaseDate = new Date(currentDate.getTime());

Here's a full kickoff example:

var currentDate = new Date();
var purchaseDate = new Date(currentDate.getTime());
purchaseDate.setFullYear(purchaseDate.getFullYear() - 1);
alert(currentDate); // Today.
alert(purchaseDate); // Today minus one year.