I have installed an x-tender modchip, and my console works fine. However prior to installing the chip, almost everytime I booted my xbox it asked for the date/time. Since installing the chip it does not and when I checked the date/time it is always the same with the year as four thousand and something.

Anyone know why this may be the case?

well, do you leave your xbox plugged in to the socket more of the time since installing the chip?
The way it works for me is, if I leave the plug out for an extended length of time then turn it back on again, it asks for date/time, wheras as long as its plugged into the wall, it does not..
Anyone else confirm this?

Yep. Or try the manual.

TOTU is correct. The XBox doesn't have an internal clock battery like the PS2. My guess is that there's a capacitor that will keep the time in the even of a power outage for a short period of time. All the units I get, I have to adjust the date/time when I pre-test them.

You don't unplug it from the wall outlet do you? Or is the outlet switched so that when you turn off the switch, it cuts the power to the outlet?