5) Seventy Calendars for an Empty Planet
A Century of Martian Calendars

The first awkward fact about proposing a new calendar for Mars is that quite a lot of people got there first.

By 2005, Thomas Gangale and Marilyn Dudley-Rowley had identified more than seventy published proposals for Martian timekeeping produced over roughly the preceding 125 years. Some dealt mainly with clocks or scientific dating; others developed full civil calendars. Many now survive mostly in old journals, conference proceedings and books that few prospective Martian settlers are likely to pack for the journey.

This is good news rather than an inconvenience. Martian calendrics is not a blank page on which every useful idea must be invented from first principles. There is already a substantial design history: repeated attempts to reconcile a 24-hour-and-39-minute sol, a roughly 668.6-sol year, unequal seasons, human habits inherited from Earth and the ordinary desire for a calendar that does not require unnecessary effort to use.

The interesting thing is not that these designers failed to agree. It is that they kept discovering the same choices: twelve months or twenty-four, equal elapsed periods or closer seasonal correspondence, a continuous seven-sol week or fixed date–weekday relationships, familiar terrestrial names or deliberately Martian ones, concentrated irregularity or distributed irregularity.

The questions have proved remarkably durable.

A crowded field before anyone needed one

There is something wonderfully optimistic about designing calendars for people who do not yet exist. Mars had no residents, schools, payroll departments or pressing administrative need for a Thursday when people nevertheless began asking how its time should be organised. The historical literature stretches back well before modern Mars exploration, emerging as astronomical knowledge made the basic problem measurable.

That is less eccentric than it sounds. Calendar design sits naturally wherever rotation, orbit, seasons and inconvenient fractions meet. Martian calendrics also occupies an unusual intellectual borderland: part astronomy and mathematics, part history and social convention, part interface design and part institutional plumbing.

Different designers therefore began from different questions. Some started with the planet: how should the calendar represent Mars's orbit or seasons? Others started with people: which intervals would humans recognise and use? Others started with the mathematics: how can a fractional year be absorbed while preserving particular forms of recurrence or regularity?

The result is not a single historical line gradually converging on the correct Martian calendar.

It is a family tree.

Levitt and Gangale: two enduring branches

One particularly important branch begins in 1954, when astronomer I. M. Levitt published “Mars Clock and Calendar” in Sky & Telescope. Levitt and Ralph Mentzer had built a mechanical clock capable of displaying and correlating terrestrial and Martian time, and Levitt proposed an accompanying calendar.

Its structure was remarkably clean. Levitt divided the Martian year into twelve months: eight of 56 sols and four of 55, arranged in the repeating pattern 56–56–55. Each group of three months therefore contained 167 sols. He retained a seven-sol week and shortened the final civil week of each 55-sol month so that the next month could begin again on the first weekday.

Several important ideas are already visible here: twelve annual divisions, near-uniform month lengths, equal common-year quarters and recurring month starts. None of them requires the year to divide perfectly into weeks or months; the design instead decides where the unavoidable mismatch should appear.

The fact that this structure remains attractive decades later is less mysterious than it sounds. Mars has not changed its orbit in the meantime. The arithmetic continues to offer the same possibilities.

The other major modern branch is associated with Thomas Gangale. His “Martian Standard Time”, published in the Journal of the British Interplanetary Society in 1986, approached Martian time as more than an astronomical conversion problem. The underlying issue was how a chronometric system might reconcile physical cycles with human requirements.

That shift matters because it moves the subject closer to civil infrastructure. Gangale's later historical work also revealed how crowded the field already was. In The Lost Calendars of Mars he described discovering earlier proposals after initially believing that parts of his work were comparatively novel. Even the idea of a twenty-four-month Martian year had antecedents.

The calendar that emerged from this work became the Darian Calendar. Darian makes a different primary choice from Levitt. Rather than preserve twelve long annual months, it uses twenty-four months, normally 28 sols long with several shortened to 27. The result keeps the Martian month much closer to the duration people associate with an Earth month. Shortened terminal weeks preserve fixed relationships between dates and weekdays, and the year begins at the northern vernal equinox, Ls = 0°.

This is more than alternative arithmetic. It is a different design philosophy.

Levitt preserves the month primarily as an annual fraction. Darian preserves something closer to its familiar human-scale duration. Neither has made a mathematical mistake; they are protecting different properties.

That tension — twelve annual months or twenty-four familiar-duration months — is one of the clearest recurring choices in Martian calendar design.

The design space widens

By the 1990s, other proposals were making the underlying choices still more visible.

Robert Zubrin's calendar, presented in The Case for Mars in 1996, gives stronger weight to orbital and seasonal position. Its twelve months represent equal 30-degree sectors of Mars's orbit rather than equal amounts of elapsed time. Because Mars moves at different speeds along its eccentric orbit, those sectors contain different numbers of sols.

The calendar therefore answers one question particularly clearly:

Where is Mars in its year?

Its months carry strong orbital meaning but less uniform elapsed duration. That is a coherent choice if seasonal position is given greater weight than civil regularity.

Josef Šurán approached the problem from another direction. His 1997 work on perpetual Martian calendars treated the weekly structure and intercalation mechanism themselves as design variables. Leap days, skip days, leap weeks and skip weeks could be arranged differently depending on which properties were to be protected.

The important point here is not the detail of his preferred solution but the fact that the week itself becomes a design choice. Should weekday succession continue without interruption? Should recurring dates remain attached to fixed weekday positions? Where should the fractional Martian year be absorbed?

Earth calendar users rarely ask those questions because the answers arrived long before them. Mars makes them visible.

Gangale and Dudley-Rowley's 2005 historical and comparative work later placed these questions inside a broader architecture of time. Scientific dating needs such things as a defined epoch and reproducible annual reference. A mature civil system eventually has to deal with additional layers: weeks, months, names, biological rhythms, social organisation and the ordinary mechanics of life.

That distinction helps explain why Martian timekeeping can be both technically mature and civically unsettled. Science already has excellent ways to describe Martian time and seasonal position. The open question is what human architecture should sit on top.

A hospital, a house and an airport do not have different floor plans because two of the architects misunderstood geometry. They have different jobs. Martian calendars are much the same.

Two later examples make that point especially clearly.

In 2008, Henrik Hargitai and colleagues described a 12 × 56/270 calendar for presenting Martian climate data. Most of its months were equal 56-sol blocks, while the final month absorbed the residual difference and the year began near Ls = 270°.

This calendar begins from a particular user rather than an abstract ideal of Martian civil society. Its question is not “What calendar should Mars adopt?” but “What structure makes these climate data easier to present and compare?”

Equal elapsed blocks are useful for that purpose. Concentrating the irregularity in one final period keeps most of the data intervals uniform. A general civil calendar might place the inconvenience elsewhere.

Same planet. Different brief.

More recently, Dennis Silin's Exodus Calendar, published in the Journal of the British Interplanetary Society in 2025, gives substantial weight to familiarity. It retains twelve familiar terrestrial month names. January through November contain 56 sols, while December varies between 52 and 54 under a repeating intercalation cycle.

The design asks a different question again: how much unnecessary relearning should future settlers be asked to do?

If users already understand January, February and March, retaining those names lowers one form of cognitive cost. The trade-off is that those familiar names now describe periods much longer than their terrestrial counterparts. Familiarity is therefore not simply conservatism. It is another design requirement with benefits and costs.

Five choices keep returning

Once these proposals are viewed together, much of the field begins to organise itself around four recurring questions.

The first is scale: should the Martian month remain one twelfth of the local year, or should it remain closer to the duration humans recognise as a month?

The second is recurrence: should the seven-sol weekday sequence continue without interruption, or should the calendar protect fixed relationships between dates and weekdays?

The third is astronomical representation: should civil periods contain similar amounts of elapsed time, or should they correspond more closely to equal movements through Mars's orbit?

The fourth is familiarity: how much terrestrial structure should be retained because people already understand it, and when does familiarity become misleading because the underlying Martian unit has changed?

Other choices matter too — epoch, intercalation, nomenclature, hemisphere and computational simplicity among them — but these four explain a remarkable amount of the disagreement.

They also interact. Choose twelve approximately equal-duration months and decisions about annual scale and seasonal representation have already been made. Choose twenty-four roughly four-week months and the annual structure changes. Demand fixed month starts and a weekly design problem immediately appears.

Every attractive property spends some of the calendar's design budget somewhere else.

This is why earlier Martian calendars should not be treated as a sequence of unsuccessful attempts waiting for someone to discover the answer. They are coherent responses to different requirements. Seven-sol weeks have precedent. Shortened terminal weeks have precedent. Twelve 55- and 56-sol months have precedent. Twenty-four periods close to four weeks have precedent. Astronomically anchored years have precedent. So do concerns about recurrence, seasonality, familiarity, regularity and adoption.

That historical depth narrows any sensible claim of novelty in the field. Interesting systems are rarely interesting because every component is unprecedented. More often the useful contribution lies in recognising that existing ideas solve different parts of the same problem, then deciding how — or whether — those ideas should be combined under a particular brief.

The history therefore does not put a century of Martian calendars on a podium. There is no useful podium. It reveals a design space in which serious people have repeatedly made different, defensible choices about what matters most.

Mars has already been calendared many times. What the history gives us is not an answer, but a remarkably clear map of the choices.