Clearly, this requires some explanation.

January 12 is my birthday, and today is July 13, 2015.

• Remaining days in January, after today: 19
• Days in February through June, this year, which isn’t a leap year: 28 + 31 + 30 + 31 + 30 = 150
• Days in July up to, and including, today: 13
• Total days after my last birthday, up to and including today: 19 + 150 + 13 = 182

How long until my next birthday, starting at midnight, tonight?

• The rest of July: 18 days
• August through December: 31 + 30 + 31 + 30 + 31 = 153
• Pre-birthday January days: 11
• Total days between today and my next birthday: 18 + 153 + 11 = 182, also.

Since the number of days between the end of my last birthday, and midnight tonight, is exactly the same as my number of pre-birthday days which follow midnight, it follows that midnight tonight is the one point in time, this year, which is as far away from my birthday as one can get, on the calendar. The fact that antibirthdays are usually points in time, rather than full days, is a consequence of the fact that most years have an odd number of days. Subtract one for my birthday (or anyone’s, except for those rare people born on February 29 — we’ll get to them later), and 364 days remain in most years. Divide this by two, and there are 182 days to fall on either side of an antibirthday midnight, for most people, during most years.

Next year, 2016, is a leap year. What will happen to my antibirthday next year, then, with its 366 days? As it turns out, next year’s antibirthday, for me, will be a full day. Why? Adding “leap day” makes it necessary to subtract two days, rather than just one, to get 364. (An even number of post-subtraction days is needed for divisibility, by two, with no remainder.) My antibirthday in 2016 will be on July 13, all day long, because there are 182 days between that day and both of my nearest birthdays — one in that antibirthday’s near past, and one in its near future.

If we don’t have the same birthday, and you want to figure out when your own antibirthday is, you can follow the pattern above, with only minor adjustments, if your birthday, like mine, falls on or before February 28. Some additional adjustments will be needed for those with birthdays in March through December, though. Why it that? Simple: my birthday occurs before February, and this isn’t true for most people. My full-day antibirthdays occur during leap years only because of this fact. If your birthday occurs after February is over, you’ll still get full-day antibirthdays every four years, but those years won’t be leap years — they’ll be one year removed from leap years, instead. Whether this means such years will immediately precede, or follow, leap years is left as an exercise for the reader.

There’s a small group of people for whom this gets even more complicated: those whose birthdays only happen every four years, on “leap day,” February 29th. Of the people I know well, only one of them, my friend Todd, was born on a leap day, and, just to be a pest, I’m going to assign him the problem of figuring out his own antibirthdays. After all, he has plenty of time for this, since the fact that he only has a birthday every four years causes him to age at 25% of the normal rate. He looks only a bit older than me, having had only a few more birthdays that I’ve had, even though he was born in 1812, and can remember the American Civil War clearly. Fortunately for him, he was still a child in the 1860s, and this saved him from actually having to fight in that war, or any other. It must be nice to have a 280-year life expectancy, Todd!

[Image credit: before turning the birthday-cake picture above upside-down, I downloaded it from this website.]