Why Do We Use Calendars? A Short Essay About Time
Why do we use calendars? This thought crossed my mind recently when I created a date-time library for my project. It began as a small set of utilities — ISO formatting, basic date arithmetic, and conversion — but quickly grew into its own library. Not because there aren’t already plenty of date-time libraries, there are; not because I can do it better, I can’t; simply for the joy of it. And then I asked myself: why do we use calendars? Time is Absolute Yes, I know, time is relative. The faster we move, the more time slows down. What I mean is that a second is a constant, well-defined unit tied to the frequency of a cesium atom. In other words, it doesn't change. We can divide it down into milliseconds, microseconds, nanoseconds, and so on, or build it up into minutes, hours, and days and? And? Nope, that’s it. Months can be 28, 29, 30, or 31 days. A month is relative, but a day is absolute: 24 hours, or 24×60 minutes, or 24×60×60 seconds, or 24×60×60×60 milliseconds, and so on. Time is Simple From a technology perspective, time is surprisingly simple. We count the number of seconds since January 1st, 1970 (the UNIX Epoch). It’s just one ever-increasing integer, and because a second is constant, we convert that number to bigger units — minutes, hours, days — using basic arithmetic. I really didn’t think much about this before, but it's kind of crazy: we’ve reduced such an abstract and complex concept to a single number. Everything from a basic digital clock to the most advanced systems uses this simple principle. The complexity of time condensed into a single number. Isn’t that beautiful? Time is Intuitive Let’s say it’s January 1st, and I ask you to meet me in a month. Naturally, you’d think of February 1st. If it’s January 15th plus a month, that’s February 15th. But what about January 31st? Adding a month would bring you to the end of February — likely the 28th. Now consider January 28th plus a month; that also suggests February 28th. But where did the days between the 28th and 31st go? And what happens if it’s January 30th plus a month — do we land on February 27th, or slip into March? In the first case, the duration from January 30th plus a month is shorter than from January 28th; in the other case, it’s longer than from January 31st. Now, what about days? More specifically day of the year. The 1st day of the year plus 15 days is the 16th day of the year, plus 30 more is the 46th, plus 365 lands on 45th the next year (ignoring leap years). That's it. No special rules or ambiguity, just straightforward math. But why Calendars? Yes, I think I know the answer: for historical reasons. But why do we still use calendars? A year is a conversion factor; it lets us avoid saying “739,627 days since 0 BC” by saying “2025 years and 11 days” instead. But why do we still use months with varying lengths? Why not use the day-of-year like the “11th day of 2025” for instance? This isn’t even a radical idea: ISO 8601 already covers ordinal dates in the YYYY-DDD format. Think of how much complexity we could eliminate by simply moving from YYYY-MM-DD to YYYY-DDD?
Why do we use calendars? This thought crossed my mind recently when I created a date-time library for my project. It began as a small set of utilities — ISO formatting, basic date arithmetic, and conversion — but quickly grew into its own library. Not because there aren’t already plenty of date-time libraries, there are; not because I can do it better, I can’t; simply for the joy of it.
And then I asked myself: why do we use calendars?
Time is Absolute
Yes, I know, time is relative. The faster we move, the more time slows down. What I mean is that a second is a constant, well-defined unit tied to the frequency of a cesium atom. In other words, it doesn't change. We can divide it down into milliseconds, microseconds, nanoseconds, and so on, or build it up into minutes, hours, and days and? And? Nope, that’s it. Months can be 28, 29, 30, or 31 days. A month is relative, but a day is absolute: 24 hours, or 24×60 minutes, or 24×60×60 seconds, or 24×60×60×60 milliseconds, and so on.
Time is Simple
From a technology perspective, time is surprisingly simple. We count the number of seconds since January 1st, 1970 (the UNIX Epoch). It’s just one ever-increasing integer, and because a second is constant, we convert that number to bigger units — minutes, hours, days — using basic arithmetic. I really didn’t think much about this before, but it's kind of crazy: we’ve reduced such an abstract and complex concept to a single number. Everything from a basic digital clock to the most advanced systems uses this simple principle. The complexity of time condensed into a single number. Isn’t that beautiful?
Time is Intuitive
Let’s say it’s January 1st, and I ask you to meet me in a month. Naturally, you’d think of February 1st. If it’s January 15th plus a month, that’s February 15th. But what about January 31st? Adding a month would bring you to the end of February — likely the 28th. Now consider January 28th plus a month; that also suggests February 28th. But where did the days between the 28th and 31st go? And what happens if it’s January 30th plus a month — do we land on February 27th, or slip into March? In the first case, the duration from January 30th plus a month is shorter than from January 28th; in the other case, it’s longer than from January 31st.
Now, what about days? More specifically day of the year. The 1st day of the year plus 15 days is the 16th day of the year, plus 30 more is the 46th, plus 365 lands on 45th the next year (ignoring leap years). That's it. No special rules or ambiguity, just straightforward math.
But why Calendars?
Yes, I think I know the answer: for historical reasons. But why do we still use calendars? A year is a conversion factor; it lets us avoid saying “739,627 days since 0 BC” by saying “2025 years and 11 days” instead. But why do we still use months with varying lengths? Why not use the day-of-year like the “11th day of 2025” for instance? This isn’t even a radical idea: ISO 8601 already covers ordinal dates in the YYYY-DDD format. Think of how much complexity we could eliminate by simply moving from YYYY-MM-DD to YYYY-DDD?
