First off, use the power of the drop down in search. It's simple and no syntax to remember. This will allow you to quickly narrow your search by document type, along with a few other parameters.
Thankfully, Google is starting to add better search capabilities to their Google Apps too, by adding added syntax to your search.
You can stack your search syntax, just like when you use the Google Search Engine.
The after: syntax can limit by a date. (Also before: works too.)
This blog article is also a Google Doc, created for staff development.