A More Rational Time

July 5, 2019


Time, as anyone who worksThis is especially true for software engineering, a field that your author is well-aquainted with. with it will tell you, is BIZARRE. Though days and weeks are finefine-ISH. , once you start talking about months and years, there are some really sharp edges. This article identifies the problems, potential solutions, and if there’s anything we can do about it.

The Problem

The Year

The fault is in our starsNot just a great book by John Green, but also paraphrasing ?caeser? . Our word for month comes from moon, and in many cultures the word is still exactly the same. In Chinese, for example, it’s yue4, which even looks like a crescent moon. Or rather, our two nearest heavenly bodies, the earth and moon. If we planned our calendars by just one of these, it would be easier. Our planet’s rotation around the sun, a year, does not divide equally into whole-numbered days, as measured by the rotation of our planet. This is what causes leap years, the formula for which is byzantine and unhelpful.A year in the Gregorian calendar is a leap year, if the year number is divisible by 4 UNLESS it is valso divisible by 400 .

The Month

A lunar month (the time between sucessive full or new moons) is between 29 and 30 days. It varies because, though the moon is orbiting the earth at a continuous rate, the interplay with the sun (and therefore the phase of the moon) is more complex. It was a good enough measure for our ancestors, but hardly one hard-headed civilisations could build a calendar around.

An Example

Our curent calendar doesn’t help us in ways that we all sort-of ignore. Consider the simple question: “what date is in a month’s time?”. Imagine it is the 10th of September. Is a month’s time the 10th of October? that’s 30 days later, but is another month’s time the 10th of November? that’s not another 30 days later, as October has 31 days. So the two intervals have different durations. If I pay you a fixed amount for a month’s work, you’re probably doing a day of work for free in October. I say probably because we now need to talk about Weeks.


I love weeks. They have problemsNot least because their duration is prime! , but at least they are REGULAR. Up until the end of the year, when you find that 365 doesn’t divide by 7 equally. But for MOST of the year, you can ignore that.

It is in their interplay with months that they fall down. Not all weekdays are equal. 5 days of work, followed by the 2-day weekend is a nightmare for figuring out how many work days there are in a month.

If the 1st of January is a Monday, then the 31st is a Wednesday, and the 1st of February is a Thursday.

Month Duration Workdays (2019)
January 30 23
February 28 21
March 31 21
April 30 22
May 31 23
June 30 23
July 31 23
August 31 22
September 30 21
October 31 23
November 30 21
December 31 22
Average 30.42 22.08
A More Rational Time - July 5, 2019 - Tristram Oaten