The onset date is the beginning of your Social Security disability claim. It is the date when your impairments prevented you from working at the level of substantial gainful activity.
In our current climate for Social Security disability claims, onset dates have been under pressure from both DDS and from administrative law judges. Even very favorable judges are looking at onset dates much more closely.
At the hearing level, part of my preparation is a hard look at the medical evidence and whether or not it supports the alleged onset date.
In the January issue of Social Security Forum, a newletter for NOSSCR members, there is a good article by California attorney Peter S. Young describing the factors to discuss with your client before agreeing to an amended onset date. Read Practice Tip - Amended Onset.