If you are found disabled under the Social Security Act, Supplemental Security Income benefits under Title XVI will typically pay you from your date of filing (provided you meet additional criteria for poverty), whereas Social Security Disability benefits under Title II can give you retroactive payments up to one year from the date of filing, though there is a five month waiting period during which you are not entitled to benefits.
The waiting period consists of five full consecutive calendar months, beginning with the earliest full calendar month that you are disabled and meet the insured status requirements under Title II. (SSA Handbook § 502.1). For example, if you are found disabled as of March 3 your wait period should comprise April through August. However, an onset date of March 1 would count March as a full calendar month, thus your wait period would consist of March through July.
An exception to the five month waiting period exists for claimants who were entitled to disability benefits as a child, or were previously entitled to a period of disability benefits and became disabled again within five years following the month your previous disability ended. (SSA Handbook § 502.2).
The waiting period for benefits has been part of the Social Security Act since its inception. Although Congress has implemented changes to the waiting period over the years, an examination of the legislative history suggests an intention to set a time frame long enough to deter individuals who can work from applying for benefits, and also for a short-term injury or illness to be corrected. (Moulta-Ali, U. (2012). Social Security Disability Insurance (SSDI): The five-month waiting period for benefits. Washington, DC: Congressional Research Service.)