(WHTM) Have you ever tried to calculate your age in days? It’s not as easy as just multiplying your age in years by 365. You have to take leap years into account, and since the Gregorian Calendar we use has some special rules for adding leap days, which I will not explain here, it’s possible for an extra leap day to sneak in now and then.

But there’s an easier way. I bring this all up because my birthday is November 17, which is, coincidentally, the start day of the Modified Julian Date Calendar. The actual start date of MJD is November 17, 1858, but it wasn’t set until 1957 because it wasn’t needed because-

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Wait a minute, I’m getting ahead of myself here. In order to understand the Modified Julian Date, we must begin with the original Julian Date Calendar. The term Julian Date can be confusing since it’s easy to assume it’s related to the old Julian Calendar, which was replaced by the Gregorian Calendar.

In fact, the French Astronomer Joseph Justus Scaliger (1540 – 1609), who is credited with devising the system, named it after his father Julius. (Or so the story goes.)

The Julian Date system, simply put, is a continuous count of days from a given start point. The start point selected was noon of January 1, 4713, B.C.E., since it was a time before written records. (That line of reasoning has held up pretty well; beyond about 3200 B.C.E. there may be written records, but nothing that can be read because the languages are extinct.) This continuous count is very useful to people in a number of fields, including astronomers, historians, software designers, and people who want to calculate their age in days.

And why the Modified Julian Date? Well, as you may well imagine, if you count days over thousands of years, the numbers pile up. Right now the days are in the 2.4 millions! Such large numbers can make math a bit cumbersome, especially with early computers which, while able to do calculations at far beyond human speed, could get bogged down with large numbers. So in 1957, the Smithsonian Astrophysical Observatory established MJD, which is basically the Julian Date without the first three numbers.

(For some reason, the Observatory switched the start time from noon to midnight, with the result that Julian Day and Modified Julian Day are 12 hours out of sync with each other. As long as you don’t mix numbers from the two systems, you’re fine.)

So why did they pick November 17, 1858, as the start day for the Modified Julian Date? Because that is when the Julian Date Calendar made the big turnover from 2,399,999 to 2,400,000. This meant they could start the MJD at 0000.

We now return you to our regularly scheduled calculation…

November 17, 2022, is Julian Day 2459901, or Modified Julian Day 59900.

Using either system, If I subtract my actual date of birth from my current birthday I learn that I am 25,568 days old. From there a simple mathematic calculation will reveal how old I am in years. I shall leave that as an exercise for the reader.

Here’s a site where you can input a date and find out the Julian Day:

U.S. Naval Observatory

Incidentally, this also leaves a mere 40,099 days until the calendar gets to 2.5 million, at which point someone may want to introduce a modified Modified Julian Date…