Currently you are only able to back date a start date i.e. You can set a future start date
When you sign up for a challenge, the challenge will automatically start. If you want to start on a different date from the day you join you will need to go in a backdate the start date.
This can only be done the day AFTER the date in which you want to set as the start date.
E.g. If you want to start on the 5th of June then you will need to wait until the 6th of June and then you can go in and set the start date to the 5th of June.
I know it's a little weird. We will be working on improving this area