You're not presently able to choose the start date for a membership. Memberships are valid from the date on which they are purchased for the length of time set. This allows you to sell full memberships at a time that suits your customers.
If you do want memberships to all be taken on the same date, the best course of action would be to take payments offline and then gift the membership(s) to the necessary customer(s). You could create a short term pass that you can gift to the customer and then manually expire on the start date of the membership if you wanted to offer the option of pro-rating your memberships.
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