How to allow services of differing times to be booked on either :00 or :30?

I teach private lessons, and would like for clients to be able to book either 30 minute or 60 minute sessions with me. When I have a service of 60 minutes there is no option to book starting at :30, only on the hour. How can I gain this functionality?

I currently have 4 services.
1 - 30 Minutes over Skype
2 - 60 Minutes over Skype
3 - 30 Minutes in person
4 - 60 Minutes in person