I would like to be able to set task constraints based on dates relative to other tasks, for instance:
"taskA to start no earlier than taskB end date + 20 days"
is there a way to do it? In MS Project I only shows absolute dates
You should be able to do this by linking tasks and creating dependencies between them.
From memory, while on the Gantt screen, ensure that the Predecessor column is shown. In the example you give, let's assume TaskB is in row 24 and TaskA is in row 25 of the page. In the "Predecessor" column of row 25, enter 24FS+20d.
There is an explanation here which gives you the basics of using MS Project (you will have to navigate through a couple of pages) although it may be slightly different for different versions of the application.