You’re staring at a wedding invitation draft or a high-stakes legal contract. Your palms are a little sweaty because you realize you have no idea if "5:00 PM" is actually appropriate or if you need to write out "five o'clock in the afternoon" like you’re living in a Jane Austen novel. It's confusing. Honestly, the rules for how to write date and time formally change depending on who you ask, but getting it wrong makes a professional document look messy.
Precision matters.
If you’re working within the United States, you're likely toggling between the Chicago Manual of Style (CMOS) and the Associated Press Stylebook (AP). They don’t always agree. While AP style is for journalists who need to save space, CMOS is the gold standard for formal book publishing and high-level business documentation. If you want to look like an expert, you have to pick a lane and stay in it.
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The Brutal Truth About Military Time and ISO 8601
Most people think "formal" means "complex." That’s a mistake. In international business and tech circles, the most formal and logical way to handle dates isn't even the "January 1st" format we grew up with. It’s ISO 8601. This is the international standard that uses the YYYY-MM-DD format.
Think 2026-01-16.
It looks cold. It looks like a computer wrote it. But in a globalized world, it’s the only way to ensure a developer in Tokyo and a project manager in Berlin are looking at the same deadline. If you use 01/05/26, the American thinks it's January 5th, while the European is certain it’s May 1st. You’ve just missed a deadline because of a slash.
When you’re learning how to write date and time formally, you have to consider your audience’s geography. For general American formal contexts, the Month-Day-Year format is king. For the rest of the world, it’s Day-Month-Year.
Stop Using Ordinal Numbers
This is a hill many grammarians will die on. Don't write "January 15th, 2026" in a formal letter. Just don't.
The "th," "st," and "rd" are called ordinals. They are fine for speech. You say "the fifteenth of January," but when you write it down in a formal business header, it should be "January 15, 2026." The numbers stand alone. Adding the "th" is redundant and clutters the page. It’s a classic sign of someone trying too hard to sound formal and accidentally hitting "amateur" instead.
The exception? Invitations.
If you are writing a formal invitation for a gala or a wedding, you go the opposite direction. You write everything out. "The fifteenth of January, two thousand twenty-six." It’s long. It’s tedious to write. But that is the peak of social formality.
Time Is Not Just A Number
When we talk about how to write date and time formally, the "time" part is where things get weirdly heated. Do you use "a.m." or "AM" or "A.M."?
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The Chicago Manual of Style prefers small caps (ᴀ.ᴍ.) or lowercase with periods. AP Style uses lowercase with periods (a.m.). Most modern business emails have moved toward uppercase without periods (AM) because it looks cleaner on a mobile screen. However, if you are writing a legal brief or a formal report, stick to the periods.
Also, "12:00 PM" is a trap.
Is it noon? Is it midnight? Technically, "PM" stands for post meridiem, meaning after midday. "AM" is ante meridiem, before midday. Noon is exactly midday. To avoid any legal ambiguity—especially in contracts—always write "12:00 p.m. (noon)" or "12:00 a.m. (midnight)." Or just use "Noon" and "Midnight." It's impossible to misinterpret.
Commas Are Not Optional
If you are using the American format (Month Day, Year), you need a comma after the day and a comma after the year if the sentence continues.
"The meeting on January 16, 2026, was a total disaster."
Notice that second comma after 2026. Most people forget it. If you leave it out, the sentence structure technically breaks. However, if you use the British/International format (16 January 2026), you don't need any commas at all.
16 January 2026 was a great day.
It’s cleaner. It’s faster. But if your boss is a traditionalist in a New York law firm, stick to the commas or prepare for a lecture on "proper" American English.
Handling Decades and Centuries
Let's talk about the 1920s. Or is it the 1920's?
Never use an apostrophe to make a decade plural. The apostrophe is for possession or contractions. Unless the year 1920 owns something, keep the apostrophe out of it. It’s "the 1990s," not "the 1990's."
For centuries, the rule is to lowercase and hyphenate when it’s an adjective.
- "In the eighteenth century..."
- "Eighteenth-century literature is fascinating."
If you start a sentence with a year, you have to write it out. "Twenty twenty-six is going to be a big year." It looks terrible, doesn't it? Most editors suggest reconfiguring the sentence so the year isn't the first word. "The year 2026 will be huge." Much better.
Time Zones: The Silent Professionalism Killer
If you’re setting a formal meeting, and you don’t include the time zone, you haven't really set a time. But how do you write it?
Standard time is currently active in the winter (EST, MST), while Daylight time is for the summer (EDT, MDT). If you aren't sure which one it is, just use "ET" or "MT" to be safe. "The webinar begins at 2:00 p.m. ET."
Avoid using "GMT" unless you are actually in the UK. For global coordination, "UTC" (Coordinated Universal Time) is the professional standard. It doesn't shift for daylight savings, making it the bedrock of international scheduling.
Real-World Application: The Formal Letter
When putting all this together for a formal document, the layout usually looks like this:
The Date Line
January 16, 2026
The Reference Line
Subject: Quarterly Review (Scheduled for 10:00 a.m. EST)
Notice there are no abbreviations like "Jan" or "16th." Everything is spelled out. It feels heavy and intentional. That’s the goal of formal writing. You aren't in a rush. You are being precise.
Common Pitfalls to Avoid
- Avoid leading zeros: In most formal writing, don't write "January 06." It’s "January 6." The zero is for data entry, not for humans.
- Don't mix formats: If you start with Month-Day-Year, don't switch to Day-Month-Year halfway through the document.
- Watch the "o'clock": Only use "o'clock" with whole hours. "Five o'clock" is fine. "Five-thirty o'clock" is a linguistic nightmare.
- Military Time nuances: If you use 24-hour time (17:00), don't use "p.m." or "a.m." alongside it. It’s redundant.
Actionable Steps for Formal Documentation
To ensure your documents meet the highest standards, follow these specific steps:
- Identify your style guide: Ask if your organization uses AP, CMOS, or a custom house style.
- Strip the ordinals: Scan your document for "1st," "2nd," or "3rd" in dates and delete the letters.
- Check your commas: Ensure every Month-Day-Year date has a trailing comma if it's mid-sentence.
- Clarify Noon/Midnight: Specifically label 12:00 to avoid any possibility of a scheduling conflict.
- Standardize Time Zones: Add the three-letter code (e.g., CST) to every time mentioned if the document crosses state or national lines.
Formality is about removing friction and ambiguity. When you write a date or time clearly, you’re telling the reader that you are detail-oriented and respect their time. It’s a small thing that carries massive weight in a professional environment.