How do I reset the time on my computer without turning it on?

Take the CMOS battery out for a few seconds. That's the little watch-type battery on the motherboard. (While the machine is unplugged)

And / or use the CMOS jumper on the motherboard, and drain the CMOS that way. Your manual will tell you how. Normally you move the jumper, turn on for a few seconds, and then move the jumper back.

That will reset the time to 1970-ish.

Check the beep codes carefully. Here are the common Dell codes If the diagnostic is installed you should also be able to run the diagnostic. Press F12 when you see the Dell splash screen.

Beep Codes     Possible Causes
1 - 2          No video card detected
1 - 2 - 2 - 3  BIOS ROM checksum error
1 - 3 - 1 - 1  DRAM refresh error
1 - 3 - 1 - 3  8742 Keyboard Controller error
1 - 3 - 3 - 1  Memory defective or not present
1 - 3 - 4 - 1  RAM failure on line xxx
1 - 3 - 4 - 3  RAM failure on data bits xxx
1 - 4 - 1 - 1  RAM failure on data bits xxx