If you look at the Sticky just above your post you will see Uniden BCD436/536HP FAQ Read This First!! and in that you will see:
My new BCD436HP/BCD536HP is constantly asking me to set the time and date...what gives?
From Uniden's UPMan:
Keep the Time!
Plug in your scanner and turn it on. The first things you'll see are prompts to set the date and time. The scanner has a real-time clock with backup power that will run the clock for about 1 week. The backup power needs to charge for about 50 hours before full backup-time becomes available. If the backup source is depleted before power is restored, you will be prompted to set the time again upon power up.