IF/THEN on Blood Collection Times Post COVID-19 Test Using Date/Time

I'm trying to write an IF/THEN formula regarding time (military time) since a sample draw.

Blood collection from before the test date/time to 24 hours post COVID test is DAY 0, 24-48 hours post test is DAY 1 and 48-72 hours post test is DAY 2.
Example: A COVID test was done on 4/25/21 19:35 ($M$4 is the COVID test date/time) and blood was collected on 4/26/21 05:20. It should still be Day 0 but it's popping up as Day 2
.
I had a formula that seemed to be working but on some cells it doesn't.

Here is the formula I've been working on:
IF(AND(DAY($M$4)=DAY($A7),$A7-$M$4<1),"DAY 0",IF(AND(DAY($A7)>DAY($M$4),$A7-$M$4>=1,$A7-$M$4<2),"DAY 1",IF(AND(DAY($A7)>DAY($M$4),$A7-$M$4>=2,$A7-$M$4<=3),"DAY 2",IF(AND(DAY($A7)>DAY($M$4),$A7-$M$4>3),"Over 2 Days"))))