When a file is served, the default value for a global is the last value that was set locally. There's three ways to change a global on a served file.
1) Take the file down, change the global and re-upload it.
2) If you have access to Server Admin Console, you can run a server side script once to change the global.
3) You can have a script run on start up for each user that sets the global to the value you want. The default value never changes, but is overriden on start up.
Thanks for the clarification David - I'm going with option #2
Since you are putting today's date in this global field, you can also dispense with the field and just use get ( CurrentDate ) in its place.