Accuracy and low current consumption

Last March I commented that I had put a 12-volt MotoMeter car clock on a simple Meccano test-rig, energising it from a 9-volt Exide dry cell. The intention was to put this clock in my Morris 1100. But time evaporates and this has not yet been possible. The other day I realised that the clock was still on its test-rig. It is still functioning, has gained approximately 3 min. in six months, and the original Exide DT9 cell is not yet exhausted. This, I suppose, is where I write verb sap!