Use one date table

I'm confused as to what you're actually trying to do here - I can get you want to look at all employees that started in, say, 2019, that's fine. I can get you'd want to look at all training that was done in, say, 2020, that's fine, but I can't really think of any sensible use to use a single slicer here unless you're wanting to say "I want to see all training done in 2019 by employees that started in 2019". Really think that any use for this sort of thing would use two slicers - what exactly do you want to do with just one slicer?