Further to Jerry's pointer, see this thread for a description of how you can set up an 'insert date' pick in your menu.
If you want (thanks to Badunit for showing that this was possible) instead of going through the steps described there you can just grab a copy of the service via this link (automatic download from Dropbox). When you doubleclick it, you'll be asked if you want to install it. You may have to go to System Preferences > Security & Privacy to click 'open anyway' if you get a warning message.
That's it. After it's installed it will be a menu pick similar to the screenshot above.
If you want to rename it, then in Finder hold down the option key choose Go in the menu, navigate to Library > Services and rename it there as you would any other file.
If you don't like going to the Services menu then assign a keyboard to it at System Preferences > Keyboard > Shortcuts.