I received a text the other day saying:
You will be paid Carer’s Allowance of about £66 every week from 02/09/19. We’ll contact you about any other money we owe you. You don’t need to contact us.
Why is it starting in advance? I get it for my disabled daughter
They need time to set up the bank transfers.
The important bit is actually the second sentence, saying they’ll be in touch about any other money they owe you. Often this is a lump sum payment from the day you applied, up to the date of first regular payment.