BeShoo said:
In fact, you do not qualify for common-law status under the immigration law until you have lived together for at least 1 year continuously (may be different if there are children), so I'm not really sure that the correct answer to this might be one year after you started cohabiting. Maybe it needs an additional page with an explanation.
Definitely do not put one year after you started cohabiting! Then the visa officer will think you moved in together on that date, and you will need another year together to qualify.
The definitions of a common-law relationship vary. To
apply as common law, you need to have lived together for one year already. But that does not mean the common-law relationship started after a year together. CIC could have arbitrarily chosen 2 years, or 6 months, or any time, as the amount of time necessary to have cohabited to apply as a common-law partner.
If you are worried that any answer you give is not what they meant, you can always attach an explanation to that form, giving more details. So in this case you could state that you started going out together on ____ date, moved in together on ____ date, and that the one-year anniversary of your moving in together was ____ date. But I think using the date you moved in together as the start date of your common-law relationship will be clear, and will be corroborated by all your other evidence, without adding further explanation at this question.