You can add the sunrise and/or sunset times to each day if you have SunCalc by Brian Savage installed. In the preferences file, set the variable SunCalcPath to the : or / terminated path of the SunCalc application (do not include the name 'Suncalc' at the end of the path).

The settings for SunCalc (the longitute, latitude, timezone, and hour format) are read from the environment variables as described in the SunCalc documentation. If you have the rexxtricks.library installed, FWCalendar will be able to take into account changes to and from daylight savings time (change the AdjustDST variable if your location doesn't participate in DST time changes).