I know this is an old post, but this is one of the few posts about this problem, and I just ran into this problem on my Police 911/2. In most cases, I'd guess it means the Timekeeper RTC battery is dead.
AFAIK, the Police 911/2 Timekeeper hasn't been dumped... BUT, luckily in my case (and in whoever got r5g's case probably), the game was an upgraded Police 911 -> Police 911/2, which means you can start with the Police 911 Timekeeper, which is dumped, and the game will upgrade it to Police 911/2 (it will say "Conversion Succeed"). I don't know whether dedicated Police 911/2 boards will also perform the upgrade if you start with a Police 911 RTC. I also don't know whether the upgraded Timekeeper data will work on a dedicated Police 911/2, or just an upgrade (my guess is both)... but I'll be submitting the dump to MAME either way (drop me a PM if you need the dump before it shows up in the MAME sets).
DogP