Question 13b asks which date you entered a common law relationship. My gf and I moved in together on July 11, 2020.
So would we put July 11th or 12, 2021 as the day that we started our common-law relationship?
Don't think the specific day 11th or 12th matters much (I think actually July 10 is correct, because that's 365 days - in the same way Jan 1-Dec 31 is one full year).
Note, there was a case here of a couple that did as you did - that is acc to instructions saying entered into common law one year after day they moved in.
BUT: they got a PFL saying they didn't meet the common law requirement (i.e. the officer thought the date meant the day they started living together. I seem to recall it was just an error and their explanation worked.
But out of an abundance of caution, I'd add a SHORT letter of explanation that you moved in together on such-and-such and therefore you are putting date one year after start of cohabitation.
Certainly not needed for couples together more than two years.