Believe it is for reporting purposes. Let me break it into steps for you
Step 1: Create a Mapping module dimensioned by Custom Time and insert one line item formatted as Native Time Period
Step 2: Map it manually by selecting Month 1 against Jan, Month 2 against Feb and so on and so forth
Step 3 : Go to Target module and use the LOOKUP in your formula such as
Source Module. Data Line item[Lookup: Mapping module. Line item]
Note: If your custom list resets after Month 12 (from Month 12 to Month 1 Next year ) then you may need to write a logic in your mapping module instead of doing it manually which will map it months of current year.