Digital logbooks invariably help a lot in this.
Effectively they draw a great circle route from departure to arrival field and divide the distance over duration, thus incrementally calculating down to the minute when your great circle track encounters night time according to FAA/EASA regulations.
Some logbooks even log takeoff and landing time, enabling further calculation of ground time in Day/Night calculations.
There are plenty of websites giving you location based times, such as
sunrise-sunset.
For a route calculator have a look at this
crewlogbook utility