If you're thinking of a clock like the PSR-800 has, then there's a problem: the PSR-500 doesn't have the hardware necessary to maintain a clock, especially when it's "turned off".
The PSR-800 has clock hardware that is maintained (via a few microamps from the batteries) when the scanner is powered down.
If any kind of "clock" functionality was added to the PSR-500 firmware, you'd have to set the clock every time you turned the scanner on.