The Martian calendar system in The Iamos Trilogy

In preparation for the last book in the Iamos series finally? coming out?? soon??? (we can all hope and pray), I am getting together some supplementary material for the series to help readers out, especially those who want to be able to jump into the final book as soon as One World is out without having to start over from the beginning.

Right now I’m working on detailed recaps for each book in the series, which I will link to on each of the books’ pages on the website, and will also include a link to at the beginning of the One World ebook. There will obviously be spoilers on all of those pages, so make sure you don’t read them unless you’ve already read the books.

There are no spoilers on this page, though! While I was working on the recap for Fourth World, I thought it might be a good idea to have a page where I explain the Martian dating system in detail. This was originally posted on my Patreon a while back, so if you’re interested in this kind of stuff, please consider becoming a patron—I share a lot of details like this and my brainstorming process over there!


The Martian colony (not Iamos, but Isaak’s people) uses a modified Gregorian calendar to keep roughly in line with Earth. While it will typically not be the exact same day on Mars as it is on Earth, this allows the dates to line up closely enough that the years switch over at the same time, and people who live on Mars will have a birthdate and age that corresponds with what people on Earth are used to.

The Martian colonial calendar is broken into two “annums” for every complete orbital rotation, i.e. true Martian year. (Yes, grammatically this should be anni, but when has colloquial shorthand ever been grammatical? When consulting with my beta readers, everyone agreed that “annums” sounded more like what English speakers would wind up saying, even if it’s “wrong.”) These annums line up with the year dates on Earth, so the annum that Fourth World takes place is 2073 C.E. on both Earth and the Martian colony. Different Worlds takes place over a full orbital rotation, beginning partway through the annum 2073, encompassing all of 2074, and ending in 2075.

If you’ve read Fourth World, you’ll remember that the book begins in April-II. Because a Martian year is slightly shorter than two Earth years (687 days), you can just about fit two of every month into one Martian year. That means for the first go-round you’ll have April, May, June, etc, and for the second you’ll have April-II, May-II, June-II. This is similar to how Adar can be written on the Jewish calendar, which includes an extra month during a Jewish leap year.

Because the Martian orbit is not exactly two Earth orbits, the dating is slightly different. Every month is thirty days rather than alternating thirty and thirty-one days. The month of February in I-years contains 27 days. In II-years, the month of February is omitted, making it a leap month. This catches the Martian calendar up with the Earth one.

One of the main characters of the series, Henry, was born in February. This means that his birthday only comes around every other annum, similarly to how people on Earth who are born on February 29 during a leap year only see their birth dates every four years.

Even though the Martian orbit lasts almost two Earth years, you only get one of each season per rotation. So that’s why at the beginning of Fourth World, Isaak calls April-II their “spring April.” They also get a “fall April” (or “autumn April” for non-Americans) every other annum.

When I was writing Different Worlds, I had to keep closer track of dates and seasons than I did in the other books, so I wrote out a detailed timeline. Here’s an excerpt of some of the notes I made:

Martian year is 687 days (30 days x 22 months + 1 27-day month)
Summer lasts 6 October-II, 2073 – 8 April, 2074
Fall lasts 8 April, 2074 – 3 September, 2074
Winter lasts 3 September, 2074 – 9 March-II, 2075 (no February)
second New Year’s is in the winter

(Since New Year’s Eve factors into Different Worlds twice in the story, I needed to know which season the first one fell in, and which season the second one fell in. 😂)

Hopefully this breakdown of the Martian calendar/dating system/orbit and seasons makes sense and is helpful to anyone reading The Iamos Trilogy!

Leave a comment

Your email address will not be published. Required fields are marked *

Prev Post Next Post