First day of period.

Hello, i am the clearblue advanced fertility monitor's user for two months. I realized for both months, last month Aug and this month Sept, when i keyed in my first day of period, lets say it was on the 2nd Aug, the monitor stated on the 3rd Aug. On the month of September, this happened as well. I keyed in 1st of Sept, my first day of period, the monitor stated on the 2nd September.
Is this normal? Thank you so much.