changing calendar?

Is there a way to change the period and ovulation dates?  I know when I get and end my period to log. But every month they project it to come again in 18 days. And every month it comes in 21. And then it says I ovulate the day after my period ends, when in reality I ovulate about a week after my period ends. Is there a way to correct this?  As it's not a very helpful period and ovulation tracker when it's always off.