"If your application is approved, your first Social Security benefit will be paid for the sixth full month after the date we find that your disability began.

For example, if your disability began on June 15, 2007, your first benefit would be paid for the month of December 2007, the sixth full month of disability.

Social Security benefits are paid in the month following the month for which they're due. This means that the benefit due for December would be paid to you in January 2008, and so on." from the ssa.gov website

So...your SSDI should start when you were no longer working and your doctors deemed you disabled, right? And when you applied.