To enable your members to override the default membership start date when purchasing a membership go to Settings> Membership Plans and edit the relevant membership plan.
​
​

Scroll down to the the section called Default Start Dates for Memberships

When a member purchases a membership via an online form or the member app, the start dates for their membership will default automatically to the date of purchase/approval.

Here, you can tick a box to allow your members to override the default start date when purchasing through the mobile app.
​

You can limit how far the start date can be moved into the future. To do this, simply enter several days, weeks or months.
​
Members can choose their membership start date when purchasing memberships through the App, Member Portal or via a Waiver.