Retirement tax questions

One more question - the 2020 1098-T I received from the college reports the payments they received from us for the 20-21 school year tuition paid. It was money received from aug - dec 2020.   However, in Jan-March of 2020 I paid $21k for 19-20 school year expenses.  I don’t see that reflected on the 1098-T.  Are eligible expenses for the AOTC based on calendar year or school year?  Thank you so much.