Open a terminal window. You have to have git installed.
cd ~/Library/Application\ Support/Alfred/extensions/scripts
git clone git://github.com/claaslange/alfred-timestamps.git
Open the Alfred search bar and type
timestamp
to get the current timestamp. Growl will display a notification and the timestamp is copied to your pasteboard.
If you type
timestamp 1333231200
Growl will display the date and copy it to your pasteboard.