Does the Stop Clock make a clicking noise (could be extremely quiet) when it turns on and off or is it completely silent? If it clicks, it has a mechanical relay and it will be fine. If it's silent, it's probably a triac.
Originally Posted by David Brown
While a triac can in theory be damaged by attaching an LED switching supply to it, that's highly unlikely and I would not expect RHD to cheap out on the protection circuitry. Surviving a globe-blow event on an enlarger with transformer is far more stressful to a triac than an LED supply. More likely though is that the LED just doesn't light, not that any damage occurs.
If you have the ultimate in paranoia then you can buy an electromechanical mains relay, i.e. one with a 240V (or 115V) coil and mechanical contacts, e.g. ebay 390510321612 or similar. Let the timer switch the relay on, which will switch your load on - that will work, guaranteed, and it will not damage the timer or the LEDs. I don't reckon it's necessary, but wait to hear back from RHD before taking my advice on their product that I actually know nothing of the specific internals