Renewal of membership loses a day

Manual membership renewals lose a day. Case as follows: membership type of rolling, with 1 year or 12 months duration, a start date of 30 Apr 2011 and end date of 29 Apr 2012. On renewal the expiry date is set to 28 Apr 2013. This is 1 day short. New expiry date should be 29 Apr 2013 - i.e. one year from 30 Apr 2012 which is the first day of the next year's membership. The system seems to be starting the new period on the last day of the old period, rather than the next day. Reproduced on sandbox.