Pip review: if increase/decrease until what date does that go back to

Dear fellow members and moderators,

I did not receive my PIP review within the given 2 weeks.
I called PIP on the number given on my text message saying if pip review form not received within 2 weeks to call them.
They said they were re-sending it.
Got a text message the following day to say they have sent me ‘another’ pip review form and to return it by a given day which is 1 month from the day they sent me the texT.

Do you now if in case of increase/decrease of pip money, to what date the amount is backdated to?

Many thanks,
Best wishes