For the record, it's not JUST pulling out the battery or draining it fully that causes the internal clock to shut off. When an N900 is still new, it has a tiny capacity internal battery inside that is able to keep the clock going when the device has no battery in it, for a little while. But it doesn't age well, so after a while, that tiny internal battery fails to hold any charge permanently, and then you're stuck with the situation that if you pull out the battery, you lose your time/date settings.

Anyway, thank you VERY much to both Reinob for his suggestion, and to stoned for testing it and verifying that it's safe. This will be great for the N900s on which the clock always resets whenever battery is pulled out.

I second Garrett's question - that date-time screen would be VASTLY less annoying if you could manually set the year in advance, instead of it always starting at 2009 or whatever.