@Andreas Yep, though this is less to do with clocks and more to do with, essentially, the randomness of cellular processes (stochasticity, if you want to impress your friends). Conditions may be right, but receptors may not fire, or those receptors may not start the process, etc. This was selected for in evolution because, as you said, sometimes good times don’t last, and plants with a few extra seeds waiting until later will have a better chance of surviving potentially harsh early spring weather.
Interestingly, a very similar form of this can be found in some bacteria, most importantly tuberculous. Some cells will simply shut shut down for periods at a time, with almost no life processes. Then, randomly, they start up again. This is why it can be such a nasty disease to fight, because most ordinary medications can’t fight a cell with no metabolic processes.