So how your trust score become +1 since it's not a month that you have got positive feedback

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However I am not saying you are totally wrong. I just noticed on my first score, after I didn't bothered to calculate it. You might get more clear answers from below quote. Perhaps first score reflect after two weeks then increase by every month.
It starts changing after approximately 2 weeks, theymos uses "round_up" for months.
Yaa, its rounded up for an total month.
I think the algorithm shows it clearly!

score += min(10, round_up(months since rating))