How do I change the time on a Potterton EP 3000 clock?
Having just asked my own question about an EP3000, I think I know the answer to this one. The current time (and all your programmed on/off times) are preserved during a power outage by a little rechargeable battery that's kept topped up by the mains. But like any rechargeable battery, it's got a finite life. Yours may have died some time ago - you just didn't notice until you switched the power off.

I didn't bother trying to prise the battery out of mine (which is also dead), but based on this discussion it's almost certainly a Varta 250DK. It's an oval about 30x40, 70mm long, containing 8 x 1.2 NiCad cells (9.6 volt total). 

You need a bit of confidence (and brute force!) to prise the programmer off its back-plate which is screwed to the wall. It's "half-hinged" on the top, but there's a moulded "retaining latch" on the bottom right. Look from underneath and you'll see a 5mm "slot" about 5cm in from the bottom right. Push a screwdriver hard up there to free the latch, and simultaneously pull/prise the bottom edge of the programmer outward and upward.

I can't say exactly where you'll get a replacement (I doubt Varta still make them). The important thing is it should be an 8-cell 9.6v NiCad (don't use Li-ion or whatever). If you take the old one in to Maplin or similar they might be able to supply something compatible.

Other options are: Expensive - pay an electrician/gasman to come and deal with it; Cheap - just live with it and get used to reprogramming the thing whenever you've had a power cut or turned it off for some reason.