I now, this is probably a topic for discussions and a lot of people have different opinions on that, but anyway:
It seems that Coda currently counts Sunday as the first day of a week. You can see it when applying the formula =weekNumber(). According to ISO 8601 Monday is the first day of a week number – Sunday the last.
This starts to be a problem, when you create a calendar like the one in the Betterments Template
My suggestion is to apply the ISO Standard as default and maybe add an option to count Sunday as first day of a week one day.