Calculating the safe period days is very simple. You have to fill in the dates of your last period, duration of your period and the duration of the menstrual cycle. And your data is ready for the next months! The dates with white spaces in the calendar (given below the calculator) are the safe day. Generally, one to seven days of the menstrual period, and the days after ovulation are considered safe or infertile.However, remember that the calculator is purely based on the details you give, and can be inaccurate if the data is incorrect.