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.