When you add a new store and pick a new plan for the store, Around.io will try to ensure that your billing date remains the same for all the stores you pick. (This way, your card will be billed on the same date for all stores).
However, this depends on what plan you pick.
All your stores on Basic Social Monthly and/or Advanced Social Monthly will have the same billing date.
And all your stores on Basic Social Yearly and/or Advanced Social Yearly will have the same billing date.
That is, all stores on the same frequency of plan (e.g Monthly) will have the same billing date. This is to make sure your billing happens on the same day for all your stores (consolidation).