Comparing dates in JavaScript

Yes, both of these things are possible. You should convert the value of the form field to a Date object using the util.scand method (make sure it's not empty first, though!) and create another Date object for the threshold date, and then compare the two by using their getTime methods.

To subtract 3 days from a Date object you can use its setDate and getDate methods, like this:

var d = new Date(); // d is the current date

d.setDate(d.getDate()-3); // d is now the current date minus 3 days