Unfortunately, the Excel Pilot Logbook does not automatically calculate the Night PORTION of your flights (based on your Off Blocks / On Blocks times) - as some other apps can do.
To elaborate, it will not calculate the Night PORTION of a flight between, for example, 16:00 and 20:00 when sunset is at 18:00. Excel Pilot Logbook WILL calculate the Total Time to be 4 hours but it will not calculate the Night portion to be 2 hours.
Of course, it is possible to formulate an algorithm to do this, based off your on/ff blocks times to/from different airport locations around the world. We've tried. However, it is very resource-heavy and results in a much larger file size (because the file has to store every single airport latitude/longitude data). Neither of these downfalls are good for performance & usability. Sorry.
You'll need to enter the amount of Night time you want to log (per flight) manually - as you would do with any other type of hours (Cross Country, Multi-Engine etc).
Once you have entered your Night flying manually in the Logbook, the Dashboard will indeed calculate the TOTAL Night flight time from all of your flights - as the Dashboard does with every other category/type of flying.