The time denotes the Android version. Right now, the latest Android ad shows 5:00
https://twitter.com/google/status/476786466790440960
Previous Android ads show the time to coincide with the version release.
> What's new? Well here we see Android 4.4 KitKat, that is backed up by the time on the clock, 4:40. Whenever Google launch a new OS version they make sure the clock matches up. 4:30 for 4.3, and 4:20 for 4.2 (you get where I'm going with this).
http://www.isozial.so/2013/10/nexus-5-leaked-onto-play-store-starts.html
Google has had a history of doing this, as we can see with the following images:
http://imgur.com/a/TwIhP
Android 5.0 is next. Get hype!