Sayyed Muhammad Husaini Ragheb, Sayyed Muhammad Husaini Ragheb has a BA in Law from Guilan University, Iran and has also undertaken Hawzah studies in Qom. He is a Cultural Affairs director of Ethics Group of Al-Mustafa Open... Answered 3 years ago

The criterion for starting a fast is to avoid any invalidators of fast from the Fajr Azan till Maghrib Azan.
So as far as I understood from your question, you missed the due time so if you want to start the fast, you should not eat anything and continue till Maghrib