How do I format this code so the date only updates once (when the file name is "Template") and doesn't update once the file name has changed?

I'm trying to piece together two bits of Javascript code that I found on these forums, but I'm not experienced in this at all so I keep getting an error message. This is what I have so far:

this.addScript("init", "var f = this.getField(\"Today\") + if (this.documentFileName=="Estimate Template.pdf")) f.value = new Date();");

What argument am I missing in this line of code?

Thanks in advance!