There Is No Reason to Ever Choose the MM/DD/YYYY Date Format

And very few reasons to use MM/DD 📅

Soviet calendar for 1929-1930

The United States is, with a few partial exceptions,¹ the only country to widely and officially use the MM/DD and MM/DD/YYYY date formats. For example, September 20, 2026 is often written “9/20” or “9/20/2026” on forms and in informal text in that country. With apologies in advance to my American friends, I will demonstrate that this is wrong, and that everybody, including everybody in the United States, should ditch MM/DD/YYYY, mostly ditch MM/DD, and use other formats.

Problem: MM/DD/YYYY and MM/DD are ambiguous

Rather obviously, for any date less than or equal to 12, there is ambiguity as to whether a number means the month or the day of the month. DD/MM and DD/MM/YYYY are more common worldwide, so in any context where your text may be read by people from more than one country (for example, anything published on the Internet), it will be potentially unclear to at least some readers whether “10/11” means October 11 or November 10.

A small number of dates are also ambiguous with other common symbols that use digits and the slash, such as fractions. If you see “3/4” in text, it could mean March 4, April 3, three quarters, or “three out of four.” It’s usually clear in context, but not always.

(MM/DD/YY is even worse. For example, 09/12/11 could mean any of 12 September 2011, 9 December 2011, 11 December 2009, or even, in theory, weirder things like 12 November 2009. Fortunately, this format has already mostly been ditched, following Y2K.)

Of course, Americans would be loath to adopt the convention of other countries, especially anything used in Europe, just like they would be loath to abandon the Fahrenheit scale, which they prefer to defend with adorably silly arguments.² Fortunately, I am not proposing adopting DD/MM/YYYY as a matter of general practice.

Solution: Month names are the cleaner alternative most of the time

Instead of “9/20”, it is better to use one of the following:

• September 20

• Sept. 20

• Sep 20

• 20 September

• 20 Sept.

• 20 Sep

• (or any of these with the year appended at the end, like 20 September 2026)

The ordering doesn’t matter much; the convention differs between countries, but since there is no possible way of confusing “September” with “20” or any other number, you can pick whichever you like. The Chicago Manual of Style, an American style guide for what it’s worth, recommends 20 September 2026.

These are often better than both MM/DD and DD/MM, and their equivalents with YY, for several reasons:

1. Disambiguation: It fully and perfectly solves the ambiguity problem, stated above.

2. Cognitive load: I couldn’t find research on this, so let’s use introspective evidence. I'm in my mid-thirties, and I still read 9/20 or 20/09 a beat slower than September 20. Maybe it’s just me, and most people have fully integrated that the 9th month is September and read this just as fast as the long version, but I’m skeptical. “9” can mean a number (no pun intended) of other things, so you have to process it to decode it. It may also be a bit harder to do when not reading in your native language (I tend to process digits in French even when reading English, though this is mostly true for numbers with more than two digits).³

3. Typos. Maybe you meant to send an invitation for your September 20 event and mistyped it as “8/20” and now everyone will show up a month early. Much harder to mistakenly type “August” when you mean September.

4. Transcription errors: Relatedly, words are less likely than numbers to introduced errors when copied, quoted out of context, or OCR’d.

Of course, named months are not always appropriate. But MM/DD/YYYY is never the solution, and MM/DD rarely. (DD/MM/YYYY and DD/MM are somewhat better, see the multi-language point below.) Let’s cover the main objections.

Objection 1: I need to use numbers for technical reasons

ISO 8601 has got you covered. This standard recommends YYYY-MM-DD, e.g. 2026-09-20 I have no particular preference for hyphens vs. slashes, so YYYY/MM/DD seems equally fine to me. Dates formatted like this are good if for some technical reason you absolutely need to use numbers. They also have the nice property that they sort alphabetically, which makes them good in file names, for instance.

Sometimes you may need numbers for technical reasons but not with the year, e.g. file names sorted alphabetically within a folder called “2026”. This is one of the few legitimate uses of MM/DD. It does not rescue MM/DD/YYYY, however.

Objection 2: Using month names is language-specific

Maybe you’re going to display an expiry date on a product that will be sold to people variously speaking English (September), Polish (wrzesień), Finnish (syyskuu), and Georgian (სექტემბერი / sektemberi). Using “September” therefore wouldn’t do, just as an English speaker would be confused to read “20 syyskuu 2026.”

Numbers are indeed a decent way around this problem. However, this point favors “DD/MM” and “DD/MM/YYYY”. The reason is that MM/DD and MM/DD/YYYY are used almost exclusively by Americans, and Americans virtually all speak English (or at least enough English to know the month names). So these formats give you almost no multilingual advantage.⁴ DD/MM/YYYY is at least conventional in almost all other countries.

Objection 3: Month names take more space

This is true but weak, because you can use abbreviations. “20 Sep” is not much longer than “9/20”. You have several choices here:

• Journalistic convention: Jan., Feb., March, April, May, June, July, Aug., Sept., Oct., Nov., Dec.

• Book convention and Chicago Manual of Style: Jan., Feb., Mar., Apr., May, June, July, Aug., Sept., Oct., Nov., Dec.

• Technical convention: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec

• Canadian two-letter convention, sometimes seen on packaging and designed to work in English and French: JA, FE, MR, AL, MA, JN, AU, SE, OC, NO, DE

Besides, I find that many people are quick to jump to abbreviations when we actually rarely need them in practice. Unless you’re doing highly constrained graphic design, almost anything online has infinite free space. (And if you’re doing some highly constrained graphic design, it seems likely that you won’t have space for context and should therefore avoid ambiguous numbers.) Abbreviations are fine for quick note-taking, but are rarely worth it in most publishable text.

Objection 4: MM/DD reflects how people speak

Americans often say this to defend MM/DD against DD/MM. Quite obviously, it supports much better my proposal of using month names instead. People don’t say “ninth month twentieth.” They say “September twentieth.”

Objection 5: You’re attacking MM/DD but DD/MM is hardly any better

True, but it’s more provocative like this and therefore more fun! DD/MM at least has the strength of convention across several countries with no shared language, as established above.

Objection 6: Speaking of, MM/DD is an established convention, why do you want to change this

Well, I’m mildly annoyed and occasionally confused by MM/DD dates written by the Americans I interact with. This is obviously not the most important issue, but I do think this format is not particularly doing anyone any good, and it’s surviving out of habit and convention in one culturally powerful country. I agree that habits and conventions can be powerful things, but sometimes it’s worth penning a blog post to weaken them a bit.

But don’t take this too seriously. At least it’s not as silly as Fahrenheit!

Soviet calendar for 1930

1. Mostly countries that have a legacy of US administration, like the Philippines, Palau, and Micronesia. MM/DD is also used unofficially in some countries that speak English, due to the cultural influence of the US, like Canada and Malaysia. Besides English, this format is also apparently used in Ewe, spoken in Togo and Ghana.

2. Such silly arguments include “Celsius is for water, Fahrenheit is for humans” and “Fahrenheit is better because 0 °F is really cold an 100 °F is really hot.” I will write a post about this after I’m done destroying MM/DD.

3. Also, annoyingly, “September” is originally a Latin word meaning “seventh month.” Likewise for October, November and December, which mean 8th, 9th, and 10th, due to the fact that the Roman year used to start in March.

4. I suppose that they might be useful if your audience is made exclusively of English, Filipino, and Ewe speakers.