Excel Question re: Time something, problem, special, create

Elliotbay

Let's say in cell A5 I want to enter the time when cell A4 has something in it.

The problem is, if(isblank(A4),"",now()), will automatically recalulate the time everytime I press enter or F9. Is there a way to create a static time? Or do I have to automate a copy-paste special - value command and have it automatically run it when a time is entered?