Why does my date change at noon instead of midnight?

Most mechanical calendar watches are not AM/PM aware--they simply advance the date every other time the hands pass forward across 12:00. Where people get confused with this is because they assume it is more complicated than that.

When setting a watch that has run down, make sure you roll the hands forward past 12:00 at least once so you can see when the date changes. Then set the time accordingly so the next date change will occur at midnight, not noon.

Digital watches do not have this confusion, because you have to explicitly tell them whether it is AM or PM. Watches that display 24-hour or military time also are clear on where in the 24 hour day the time is. But most watches display a simple 12-hour representation of the time--leaving it up to the wearer to understand which half of the day they are in.