Ok, I think I understand, let's see if this works for you.
Create a module (I called it SYS Global) where it has two line items (Current Period and Current Period -1). My models' current period is Jan 18.
In your module with transactional data, create your YearToDate formula and YearToDate - 1. But on the on the YearToDate -1, remove the TimeScale (set it to Not Applicable) - this saves space because you don't want it changing everything month that is toggled. The YearToDate - 1 formula will be: Year to Date[LOOKUP: SYS Global.'Current Period -1']
Please be careful in using TimeSum as TimeSum is really only supposed to be used when you are not using Time Scale in the module as TimeSum may have performance issues, especially with models at scale. Also, in your formula, you should hardcode the parameters as next year you will have to go through and remember which line items will need to be updated.