I'm pretty sure it's by calendar year. I had to call CRA (Canada Revenue) and the agent counted days with me during the calendar year.
You don't necessarily have to rent it out. Bringing proof you need to return to the USA, however, may help make the bumps easier. If you own property in the US, bring copies of the title/mortage, etc. You can bring proof of your business as well. It will help moreso if it's a brick and mortar and/or service business that you have to be at - can't be done remotely - but not a death sentence if you can work remotely.
Other things the IO may consider for ties - your travel history (if you leave Canada within your alloted time, it's a plus. No date on passport stamp = 6 months. Date = ...the date...
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) and family back in the USA (especially dependants). They will also like to see you have sufficent funds for your stay - you can bring bank statements, credit card statements, show them the credit card. Once I even offered to call my bank at customs and let the officer hear my balance (I don't think he liked that very much though...).
Also renting it out will create revenue in Canada - and there's CRA again
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Plus you've made income in a foreign country. IRS loves that.