Canadian Law (IRPA) requires permanent residents to reside for a minimum of 730 days/2 years immediately preceding the application for a PR card. (When you renew your PR card.) Canadian law counts day spent entering and exiting Canada as one day.
Based on you saying you arrived in April 2015, you PR card would be valid until April 2020.
You must arrive in Canada ON THE DAY IN APRIL BEFORE YOU ARRIVED by the year 2018. (April next year.)
The additional 15 days you have initially will be useful if you want to take day trips to the US etc.
However, I would recommend moving as early as possible so incase there are any circumstances that would require you to leave Canada, you can do so and still be in compliance.
And having worked carefully to spend the bare minimum amount of time in Canada, you can spend that time wondering why you want Canadian Permanent Resident status, if you plan on spending the minimum legally necessary amount of time there.