When using clock scan on only a year/month or year the reformatting will default to the current year or current year/month.
% set date [clock scan 2009 -format %Y]
% puts [clock format $date -format %Y]
% set date [clock scan 2009/09 -format %Y/%m]
% puts [clock format $date -format %Y/%m]
As far as I can tell this is not system dependant. Tested on Ubuntu Precise, Fedora 17 and Windows 7, with patch level 8.5.11.
The expected behavior is to show the year or year/month that was originally scanned.