Bay Area cars with red/white numbered placecard in their rear window. What are they for?

Hi I've been seeing a ton of cars lately with these numbered placecards in their rear window. Its a white number on a red background and the number seems to correspond to the current/next month. I thought it was some sort of parking permit but I'm seeing wayyyy to many cars with these things even up in Tahoe. Any ideas what they are for?Bay Area cars with red/white numbered placecard in their rear window. What are they for?
It's a temporary registration.





Basically the number, like 3, is the month (March, for 3) that the registration will expire. The sticker on the license plate may say Jan 08 (which is now), but that sticker gives them 2 more months to renew.