With a wish to be overly pedantic, if you can keep a somewhat steady course during the day, or anchor, and pick the mid point between sun set and dawn you don't even have to know the day of the year, time of the day, or anything time related.

I was mostly confused by the wording. I didn't know 'modulo' could be used like that, but apparently it has a colloquial meaning of "except for the variation accounted for by" and now the phrase makes sense to me.