When you are awarded Social Security disability benefits, you do not receive benefits beginning on your onset date. Rather, there is a 5 month waiting period before benefits begin. As a practical matter, it is often a 6 month waiting period, because the waiting period is 5 full calendar months of disability. So if your first month of disability was a partial month, that month doesn't count towards the 5 month waiting period.
The waiting period is often a surprise to claimants. It is particularly noticeable for those claimants who apply for disability benefits shortly after stopping work, and have their initial claim for benefits granted. For those claimants, there are often no retroactive benefits at all due to the operation of the 5 month waiting period.