Hello Paul,
My 6kL has always done the same thing for me if I'm understanding your problem correctly. I am still using the Sharp Rom that it comes out of the box with. I'm not completely sure from your question- does your Z actually lose it's time and date or does the time/date program pop up still showing the correct time and date?
Whew, I just read what I just wrote and I've managed to confuse even myself! Let me approach this differently:
For me the time and date program pops up first thing after the default/splash entry screen (the one that says something like "wait 30 seconds" or some such). However even though it pops up it still has maintained the same time and date that it had prior to my reboot. Well to be more accurate it shows the correct date and the time except it seems to lose about 2 minutes during the reboot cycle. So for instance if I checked my date and time and it showed December 16, 2004 4:44am and I immediately rebooted then when the time/date program popped up right after the splash/entry screen after the reboot my date and time would read something like December 16, 2004 4:44am. But of course now the correct time and date would be December 16, 2004 4:46am. So apparently the clock does not continue to run during the actual boot process.
I haven't tried it myself but I've heard of ppl that were going on vacation etc. and were not taking their Z with them, (now WHY would anyone do that?? it's like going on vacation and not packing clothes!!), rebooting thier Z6k and after it shuts down and begins booting again they hit the hard reset button and pull the battery for storage. (If you don't do this the Z6k WILL completely drain the battery and it will forget a few of your settings.) If you chose to shut your Z down completely this way my guess is that when you booted it upon your return that it would still show the same time and date from when you shut it down and you would have to update the time and date to whatever is correct.
Now if you are just doing a normal reboot and the time and date window appears as per normal on a reboot and the time and date have reset back to it's defaults, (whatever it shows the first time we ever boot our Z's), then I believe that something is physically wrong with the unit and you probably should contact Sharp.
Having said that there is only one other thing I can think of that might produce a reboot where your Z loses its time/date completely. If the Z6k has the PC equivalent of a CMOS battery (and I'm not sure at all if it does), then you might want to make sure you have completely charged your Z, (I recommend 24hrs), before you attempt to contact Sharp.
I apologize for how mukky this post was, usually I pride myself on being clear and understandable in my posts and this one was just AWFUL!
At any rate, hope it helped! We all want your experience as a New 6000 owner here on the ZUG to be a great one so if you have any other problems or questions feel free to ask. In your families particular case I can go a step farther and say if you have questions just email me I'm not the brightest bulb on the block but if I don't know the answer I can usually point you to where you can, or if you don't mind the long distance charges you guys should have my phone number so don't hesitate to call!
Take Care,
-NeuroShock