fajr

Making qadha due to eating sehri after fajr time sets in

Q: On the first day of Ramadhaan I did not know about the 18 degrees fajr time so I ate suhoor after fajr entered according to 18 degrees fajr time. After the first day I found out about the 18 degrees fajr time and how its more accurate than 15 degrees so I started following that after the first day. Regarding that first day, since I did not know about the 18 degrees fajr time, do I have to make it up? And do I have to make it up immediately after Ramadhaan or any time up until the next Ramadhaan?