Welcome!
Yeah its a really simple problem, there is an internal battery that gives power to the memory, when the battery runs out the DC forgets the date when you turn off.
Its the same type of battery that goes in the VMU. To replace it open up your DC, look at the board at the front where the controllers plug in, there will be the battery, get it out with a flathead screwdriver (may be a bit hard as it is sometimes glued on) and replace with a new making sure the connections are still the right way round - you want the positive side facing away from the controller ports.
Hope that helps, if you need a picture just ask and i'll post one.
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