Organisms use biological timing mechanisms to synchronise life-history transitions to annual environmental cycles. For species living outside the equatorial zone, day length change is a widely used external cue for seasonal biological clocks. This paper builds on recent developments in understanding the neuroanatomical basis of day length measurement (photoperiodism) in mammals, by taking a modelling approach to the molecular readout mechanism. We find that, while a circadian clock based system can drive day length dependent changes in the amplitude of a seasonal output (in this case production of the hormone thyrotrophin), the inclusion of a positive feedback based amplifier mechanism generates photoperiodic transitions that more closely match experimental observations. The analogies between our model and those proposed for boundary generation in developmental biology are briefly discussed.