CIC doesn't clearly say that the 6-month period is continuous or accumulated. It may be dependent on the immigration officer's discretion in Buffallo.
See the link
http://www.dfait-maeci.gc.ca/can-am/menu-en.asp?act=v&mid=5&cat=68&did=398
See the link
http://www.dfait-maeci.gc.ca/can-am/menu-en.asp?act=v&mid=5&cat=68&did=398