It would be really useful as part of a complete VFR planning solution to have the facility to calculate sunrise and sunset times for your current GPS position and for other airports or locations, as can be done for example with Garmin 296. Formulae accurate to a minute or two (at least between -72 and +72 latitude) are freely downloadable from NOAA. For now I've coded them in MortScript as an add-on, but it would be a really useful addition to the product (at least for me as the pilot of a strictly Day VFR permit machine).