The anniversary of your birth may not always fall on the same day. A year is actually 365 and 1/4 days long. If you were born before 6 AM, this doesn't affect

The anniversary of your birth may not always fall on the same day. A year is actually 365 and 1/4 days long. If you were born before 6 AM, this doesn't affect you. But for everybody else 25 - 75% of the time, the anniversary of their birth actually falls on the day after, depending on what time of day you were born and whether it was leap year and whether you born before the end of February (where we get that extra day every four years). And then throw in daylight savings time. Of course, the reality is that sticking with the same day is good enough, just as having some arbitrary number describe noon. 12 is, after all, an arbitrary number too. The important thing is that people agree on what these conventions mean and that they help mark time.