I see what you were trying to say, but it would be based on growing degree days. During July thru August, they have very long growing degree days, because they have longer daytime sunlight.
As of today their daytimes are getting very short, but they have already surpassed the growing day this crop .
I am 13 hour drive away and our growing degree day are only different by 15 mins. at this point. but they were as much as 1 hour more a month ago.
Hope this makes since, you just don't add days because of they are farther north.