When a membership expiration date is set on a donation page with monthly installment payments, each installment payment renews the membership and adds the membership length onto the end of the current membership.
For example, I have a donation page with a membership type added to each donor and set to expire in 12 months. If I pay my first installment on 1/1/2016, I would initially have a membership expiration date of 1/1/2017. But on my next monthly installment 2/1/2016, my membership would renew and have a new expiration date of 1/1/2018. By the end of the year, my 12th installment would generate a membership expiration of 1/1/2028.
The expected behavior is that the membership expiration date would remain at the initial 1/1/2017 unless an installment payment fails.
Gotcha - so instead of a monthly membership you are looking for a yearly membership paid in monthly installments. This is something we will consider but does not have a timeline at the moment.
However we are paying close attention to the most popular suggestions so please share and encourage others to follow and comment!