There's no formal definition of MRR. Do you count transactions refunded as fraud after the fact? What about transactions that were partially refunded by your support team? What month do transactions that take more than one day to settle fall in? Of course that needs to be timezone-adjusted as well.

What about the case where recurring transactions fall on the 30th and 31st of each month? They'll be processed on the last day of the month, including February 28. But depending on your time zone, they might actually happen on the first of the subsequent month in _your_ time zone.

You could instead sum up recurring subscription amounts. Do you count trials? What about users who have credit or a balance on their customer object? Are you counting users whose subscription is set to expire at the end of the billing period, but who have already paid for the month?

If you ask 20 people how they should be calculating MRR, you'll probably get at least 10 different answers that produce 10 different numbers. That's why Stripe doesn't offer this as an API.