Backdating - paying benefit for a period before we received your claim - is possible in limited circumstances.
Don't delay your claim though, even if you can't get the evidence we need. Claim first and provide the evidence within one calendar month.
How long we can backdate for depends on which benefit it is and whether you are pension-age or working-age.
You usually need to show us 'good reasons' for why you didn't claim sooner.