I’m assuming you have the data in other data modules dimensiones by Time.
Since you have time as a dimension then you can just add the numeric value you wish to display as a line item and the data for each year will just be available without any special formula. If you add Half Year to the time setting (if it’s not selected ) then you will automatically have values by half year as well. Target module should have Time Scale of Half Year, this way you will have half and full year.
If for a specific reason you really need to have a formula to retrieve data by a specif time, then you can create a System module that has 2 line items, current year and prior year (use a formula to calculate these values)
in your main module you can reference the current year values by a formula like ...
Einas "Give a Man a Fish, and You Feed Him for a Day. Teach a Man To Fish, and You Feed Him for a Lifetime"
If you don't want to write the formulae then you have to have Time Scale in your Target module as well ( but at year level) and then refer the data from source module. Even if you do that you will not be able to calculate variance without formulae
It will be something like this
But as @einas.ibrahim suggested you simply create a module without Time and use LOOKUP to pull value and then find the delta between the two. Below module FYR
To your second question if you want to get Half yearly values as well you have to enable that aggregation first in your Time settings and create two additional line items or howsoever you wish to publish those.