I'm thinking of trying to build a timer that will cycle once a year with as little drift as possible, or at least some way of adjusting the drift. The mist generator/repeaters seem to be a good sart, but they barely last any time at all. Anyone have any ideas?
Try a map with a freeze-thaw cycle on the outside and an aquifer (or having prepared a sufficient supply of water within an internal cistern).
Have water drain from the internal water-source down a short shaft and to the outside. When the outside freezes, this will form a plug and stop the draining, the shaft will fill and overflow into a mid-level side-shaft which contains a pressure-plate that can detect this and do 'something'. And/or detect when the thaw occurs, the plug melts and the water drains out.
Design to taste, local geography and other conditions. (While I'm envisaging draining out of a cliff face and the water wandering into a handy river-bed, it could even work on a perfectly flat map, if you drain into a deepish pit and make sure that the warm-weather flow to the outside is pumped up and out. In fact, that could be the perpetual source (whilever water is draining outside) to keep the internal cistern supplied, though watch out for evaporation if there's no other means of replenishments.
If you're really short on water, you could modulate the cistern flow with a shorter-term on-off gates ending pulses of 'test' water, and lose a bit of resolution (but not miss the change of season as and when it happens). The problem of 1/7th evaporation might be an overriding factor in that, though.