The PB12" uses the same tech as the iBooks (G3 and G4) for storing PRAM settings, including date and time. These models use a small on-board capacitor to hold the PRAM for up to about 20 minutes when the main battery is removed or fully depleted.
This is designed to typically allow for a battery change, but nothing long term like the half-AA's used in the desktops or the coin type found in 15", 17" PowerBooks and the late '05 G5.
I wouldn't go through the trouble of changing the capacitor unless you're having booting related issues or other power management weirdness. If your main battery is too old to hold a charge, then it would be worth replacing that first, which will also bring portability back to the machine.
Other than Power Management Unit (PMU) related troubles, the only other reason to change the capacitor would be if you were to inspect the board and find the capacitor has begun to rust and/or leak then you'll want to remove it before any further damage occurs.

1. This is the capacitor you're looking for. This particular board is an iBook G3 700Mhz (16MB VRAM Opaque), however all PBG4 12" models use the same rated capacitor (0.33F, 5.5V). This one looks pretty good, but I spotted a tiny amount of corrosion in between the two inner cells at the front of the pack here.

2. Here is the underside of the board. There are two solder blobs where the pins of the capacitor push through the board (seen to the left of the "D18" and directly below the "Q55", which is blurred in this shot). Heat these one at a time with the soldering iron and pull the capacitor pins out from the other side as the solder melts.

3. Here is the suspect capacitor once removed. I was unable to see the extent of the corrosion until I had removed it from the board, but you can see this one is toast.

4. Here I have soldered on a new, replacement cap and all should be okay in the world.
Unfortunately for this particular board, it wasn't. The corroded capacitor caused irreparable damage to the PMU and I was not able to revive it (I tried a few other component-level repairs). Ultimately I had to replace the logic board to get the machine working again.
I have replaced three of these capacitors now, one on my Blueberry clamshell iBook and two iBook G3 boards. It's not a difficult job as it seems it was designed to be replaced by hand when needed. You'll just need a basic soldering iron and a little fresh solder to lock in the new pins.
The capacitor you're looking for which suits all model PowerBook G4 12" and iBook G3 and G4 (12" and 14") is;
"5.5V 0.33F H Type Terminal Coin Super Capacitor"
I bought a pack of 10 on eBay out of HK for AU$12 (free shipping).
Long story short though; Replace the main battery if it's not holding a charge and you shouldn't lose your date and time.