I know how important it is for a person to keep his dates straight. While in college at Brigham Young University, I once had three dates in a single day. Keeping my dates straight was simplified by the fact that all three of my dates were named Ruth. If only it were as easy for genealogists to keep their dates straight. We are blessed to live in reasonably rational times.
Humans reckon time by celestial bodies.
Our year is based on the cycle of the sun through the seasons. The month is based on the movements of the moon. Don't ask me why we picked a celestial mechanism to calculate time. Maybe it is because the astronomers are in charge and want to keep plain folks like you and me from understanding. At least as far back as the Romans, this problem was addressed by somewhat arbitrarily adding a few days here and there.