Since the human body is not a machine and the processes that make ovulation observable do not adhere to exact calendar dates, the symptothermal method does not set ovulation on a specific day, but declares it afterwards as "has occurred" – but at what exact time of day or day one cannot say.
However, in order to make it manageable for our clients (and also to be able to make prognoses for the following month), trackle assigns the ovulation event to a certain day. This is the day before the first higher measurement, on which ovulation is statistically most likely to have taken place. This is irrelevant for the contraceptive purposes, because here it is only important that it has occured. The time buffer to ovulation is sufficient in any case.