It's true that you don't have the rights and privileges of common-law status until after a year has passed, but the immigration form really wants to know when you got into a continuing relationship (some IOs might interpret this as "life-long"). While, you could state that you entered into a life-long relationship on the first day you met, this would be a little hard for many people to believe. You don't really know the person until you been with them for a little time, so it's doesn't seem to make sense. (Arranged marriages are an exception, but those are harder to defend. It depends on your local and family culture.)
When I brought by partner across the border a couple of months ago, the IO wanted to write that our conjugal relationship began on the day we met, but there's no way I would claim that, even though we'd been emailing and IMing for 10 months before we met in person. During our third visit, I stayed for just under a month, and we acted much like a married couple. I am claiming that this was a way of "trying out" our relationship, so I'm dating the partnership from the end of that "trial living together" period.
From what I've read, claiming that you entered into a long-term relationship the first day you met will be met with scepticism at the very least, but you've definitely been in a marriage-like relationship since you started living together so I think this is the best date to use, even though you are not legally a couple for non-immigration purposes on that date.