If you're not flying too far, or only flying from A-A, try:
www.cmpsolv.com/los/sunset.html
For calculating/logging the proportions of day/night hours for a flight beginning during daylight and finishing during night time or vice versa:
The easy way is to buy SkyLogPro
www.skylogservices.co.uk - it has a strapped on
(rather than built in) day/night calculator. Because it only allows you to enter chock to chock time, it has to make some assumptions regarding the duration of taxiing time.
The other way is free, tedious, but accurate:
www.teamsamas.com
"This page will calculate combinations of the following:
Great Circle distances between two points.(nautical/statute/kilometers)
The initial true course from A to B.
Sunrise & sunset times in UTC for point A and/or B.
Total voyage time & day/night flight totals required for pilot logbooks.
Average groundspeed (nautical/statute/kilometers) /hour"
The difference in results for the last two methods above is in the order of less than +/-3mins, for taxi durations of 5-10 minutes.
If you do use the second method regularly, building a database of airport lat/longs speeds things up and makes the job more accurate. Speaking of which, does anyone have any links to airport nav data in spreadsheet or csv format with lat/longs listed as 'DDMM.M'? I've started something but the source data is pretty rough in places.
RodgerF's recomendation looks good too. Thanks.