Hi @try and welcome to the community!
You need to add a calculate type question, which you can see how to do it in this help article: Calculate Question Type β KoboToolbox documentation
And afterwards, if you want to display the calculation, you need to create a note, and display the calculation in there, which you can see how to do it in this help article: Including Responses Inside Another Question β KoboToolbox documentation
If you need further help, donβt hesitate to ask, community will try its best to help you.
@try, in addition to what @hakan_cetinkaya has advised, you could also learn how to calculate and display the days between dates by following the post discussed previously:

