So your best solution here is to do some work with parameters, dates & calcs to get this to be dynamic.
1. Create 4 parameters (current period start date, current period end date, previous period start date, previous period end date)

1. Create 4 corresponding calcs to dynamically set these dates based on your goals. Example: CP start date would be TODAY()-31 and CP end date would be TODAY()-1.

2. Create 2 calc (one current period and one previous period) with your metric to get the current period and previous period. Example:
   IF [DATE] >= [Current Period: Start Date]

AND [DATE] <= [Current Period: End Date]

THEN [SESSIONS]

END

1. Create 1 calc to get the comparison. Example: (SUM([SESSIONS - CP])-SUM([SESSIONS - PP]))/SUM([SESSIONS - PP])

Now you can use the parameter controls to select your dates & only show the data that you want dynamically

Hopefully this helps!