Why was my ovulation entered in the calendar the day before?

femSense measures and detects the post-ovulatory temperature rise that occurs due to the increase of progesterone in your body after an ovulation. 

Du to the high measuring accuracy, femSense is able to detect this temperature rise while it happens. Depending on how often and when you read-out your Patch, you might be able to catch the detection of this temperature rise earlier or later. 

After the temperature rise has been detected, femSense calculates back when the ovulation most likely has occurred. Depending on when the temperature rise occurred and when the read-out which detected the rise happened, the ovulation then might be dated back to the day before.