when does Rover accounting year start?

You are probably going to need to contact UK Customer Service to find out (or send a message via their Contact Us in Help) unless someone here happens to know. Rover staff don't come here to answer questions.

But, more importantly, in the US, Rover doesn't report any earnings directly to the IRS (your Inland Revenue). Sitters earning above a very high threshold level might get a document from Paypal. However, the vast majority rely on our own records of earnings, which is perfectly legal here for independent contractors. We don't need a formal earnings document to submit with tax forms. So, however, Rover might do the annual accounting in its system, it truly doesn't matter to you.

There should be a list of earnings, including the dates when they occurred, on your Payment History page of the Dashboard. That can be used to create an April-April annual total for tax purposes very easily.

Good luck!