Module 1 has your line with Data in it. You can add a line to Cumulate across your custom time list.
Formula could look like this: CUMULATE(Data,FALSE,Custom Time Scale List)
In this case we want to get the number from month 12 it sounds like, so I would create a selector that would allow us to define month 12. This selector would make sense in a SYS module, however, in this mockup I just included it in the same grid. Then Lookup this number using a formula like this: 'Module 1'.Cumulate[LOOKUP: Month to Lookup]
As you can see I have my sum from the first 12 now displayed in a module that is not dimensionalized by our custom time scale.