Depending on what aircraft you fly there are tools to support you on the TOD calculation.
- The FBW A32NX has a TOD calculator that is synced to the FMC.
- The GNS530/GNS430 mod has a VNAV page which even more comfortably lets you enter some data based on your route and shows you the time until you need to start your descent - VERY nice!
I’m not sure if there’s something for the G1000 and G3000 yet.
The TOD calculator might be cool, but using in-cockpit tools if available is less immersion-breaking for sure 