Custom dynamic Stamp

I have created a custom dynamic stamp but it does not update everytime I use it and have to go in and resave the custom calc to bring it up to date. It's a very simp[le stamp that has my name and a date variable. The issue is the date does not update dynamically. Here's the code I used which I found in these forums. what I'm I doing wrong for it to not use today's date?

event.value = "Andy Phillips " + util.printd("mmm dd yyyy", new Date)