Public Trust Questions

As far as I know SF85P for public trust clearance goes back for 7 years record. I would like know, in case of a criminal charge, is it the date of court sentencing which would be used for question 20 in SF85P or some other prior date when the investigation actually started related to charges. In my case, it was a guilty plead, so technically there was no arrest date, but the court sentencing date was in March 2013. I am applying for jobs and most of the jobs I am looking require public trust. My case investigation according to my charge report started in year 2011, but the sentencing date was in March 2013. Not sure my 7 years are done or not.
Also I were discharged from one of the jobs after one year which required public trust clearance. How would I answer question 12, as they didn’t tell me anything, except saying I cannot work there any more. Is it possible to have public trust be done at your own via some private company?
Any explanation in this regard will be appreciated.

The questions asks" in the last 7 years have you been arrested for, charged with, or convicted of any offense?". There is no ambiguity, if your court record shows you were convicted in 2013 it is a no-brainer, you must disclose it! And no, you cannot have an federal background investigation done on your own.