记得以前一次性签证入境时是要打到期日期的。刚刚查了一下政府网站（见下），看起来10年多次往返签证的规定是每次入境都可呆上半年，这大概是图章上只有入境日期的原因吧。
"A multiple entry visa allows visitors to come and go from Canada for six months at a time, without having to reapply each time. It can be valid for up to 10 years, or one month before your passport expires, whichever is earlier. You must arrive in Canada on or before the expiry date on your visa."
http://www.cic.gc.ca/english/helpcentre/answer.asp?q=417&t=16 
