This behaviour has been observed on a live CiviCRM 4.5.8 site as well as the current demo site which is 4.6.6
When renewing a membership offline, the start date doesn't update to reflect the current membership term. An example (using UK date formats)
Membership type term = 1 year Membership Since Date = 01/07/2011 Membership Start Date = 01/07/2014 Membership End Date = 30/06/2015
So as of today, the member is in "Grace".
When you click "Renew" the membership start date doesn't update to reflect the new membership term, only the end date does. So the new dates are
Membership type term = 1 year Membership Since Date = 01/07/2011 Membership Start Date = 01/07/2014 Membership End Date = 30/06/2016
which would suggest this person has 2 year membership and doesn't reflect that they renewed this year, making membership dashboard data incorrect.
Is this expected behaviour or a possible bug?
Heather O.