At the current time memberships can not be pro-rated automatically, they simply become active from the date and time of purchase.
Should you wish to apply a pro-rated membership you could collect the pro-rated payment offline and gift the customer a pass with the same access settings and usage caps as the memberships they would then purchase on the correct date. Once the membership has been purchased on the correct date, which you would need to encourage your customer to do or you could gift them the membership and collect payment offline, you can simply go in and manually expire the customer's pass.
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