Adalo calculates dates using UNIX timestamps which traditionally are the number of seconds since UNIX was implemented back in 1970. The only difference is that Adalo handles these in days, meaning that if a typical UNIX timestamp is 1634310307 (number of seconds), the day equivalent in Adalo is basically just a moved decimal point: 16343.10307.
Knowing this you can create records in a collection called “Duration” that have the duration in this format and then add the selected duration from a dropdown to a selected date in a date picker. Here’s a very basic cloneable app that demonstrates how this could work. Note that the durations are stored as decimals.