Fixed membership type - start date incorrect

I have found an issue in (CiviCRM 4.6.38 and the dmaster 5.6.alpha) when a membership type is 'Fixed' the start date is incorrectly populated.

For example- Your membership runs from 1st September 2018 to 31st August 2019- the fixed period start date is the 1st of September, the fixed period roll over date is 1st August- If I added a new membership today I would expect the start date to be 1st September 2018 (and finish on 31st August 2019) with join date being today's date- but it is setting the start date as 1st September 2017.

The end date of the membership is still being populated correctly as 31st August 2019 (for a 1 year membership).

thanks, Sarah