Hello, after clicking extend membership and paying some fee, new expiration date calculated based on today's date but not on day, when memberships should been ended.
Example: user paid for group (90 days) today. He will a member of a group until 15/07/2019. - ok
But, if he will prolongating his duration of membership in few days (for exapmple 5 days, no matter) he will receive date of end his memberships 20/07/2019
Thus, user paid twice and receive only 90+5=95 days instead 90+90=180 days