Is it essential to take your BBT at the same time every morning? I ask cause sometimes I don't get…

Is it essential to take your BBT at the same time every morning? I ask cause sometimes I don't get off work until after 4:30a.m. and it's hard for me to get up before noon those days. Today was only the second day of me tracking. Yesterday I got up at 10:45a.m. it was 96.80 and today I was up at 11:05 and it was 96.76 does that 20 min really make that much of a difference?