Yes, you can apply to extend your stay as a visitor. If you passport is not stamped with the end date, it means you have 6 month, which is roughly September 8th 2017. You should apply no later then 30 days before the expiration date. The processing time for online application is 58 days, so I would advise to apply even earlier.
You can apply online or on paper using the official Canadian immigration web site
On the other hand, Canada used to have a little known rule similar to its US Automatic Visa Revalidation counterpart. Essentially, if you left Canada to the US (and only US), you can come back on expired visa within your original admission period.
PS. I have traveled Canada<->US multiple times, but I have never used this particular provision when entering Canada. In the US the CBP officers are sometimes unaware of its counterpart, which resulted in lengthy delays on the border for me. If you decide to travel with an expired visa, make sure you are carrying with you at least the relevant printout from the Canada Immigration Services site.