FDM systems vary considerably depending upon their age & complexity (cost!) - and also the capabilities (mainly amount & rate of data capture from the recorder). A simple one will indeed measure time from point A to point B. It can also then work with the g/speed to reach a fairly accurate t/down point. A very modern sophisticated system will use GPS to ascertain the exact t/down point (very accurately) and if then associated with the runway used (& airport) can build up an extremely useful database about long landings vs airports. If you factor in the wind as well it can be a really, really useful safety tool - just depends upon how much the airline really wants such a system......