The DOFD is the date that you first became delinquent under your account agreement, and thereafter remained delinquent until either the debt was charged-off or referred for collection.
Creditors are not required to report each and every montly delinquency. They could wait several months before first reporting to a CRA.
Thus, a first reported 30-late could be delayed several months after the DOFD.
Additionally, a delinquency is not reportable to a CRA as a 30-late until it is at least 30 days past the billing due date, which means the date it became delinquent under the account agreement was a month earlier. Thus, the DOFD will be at least 30 days prior to the reported 30-late.
For those reasons, the CRAs do not use reported payment history of a 30-late to determine the DOFD.
Rather, FCRA 623(a)(5) requires the creditor to explicityly and separately report the DOFD to the CRA no later than 90 days after reproting of their charge-off. This is recordec in your credit file as the "FCRA Compliance Date / Date of First Delinquency"
