Chicago Manual of Style 17, published 2017 (usually used for books) is in blue and AP Stylebook 2020–2022 (usually used for news media) is in orange. Black text indicates the two systems agree.

This page originally published March 2, 2016. Thoroughly updated August 1, 2022.

Date Formatting

Chicago and AP agree, do not use st, nd, rd, or th on date, even though you pronounce it. {{CMS 2017, 9.31; AP 2022, dates}}

Month Date Year

Chicago style: The concert will be February 10, 2022, at Carson Music Hall. {{CMS 2017, 9.31, 6.38}}
AP style: The concert will be Feb. 10, 2022, at Carson Music Hall. {{AP 2022, months, years}}
(AP note: If it’s this year, don’t include the year.)

Day Month Date Year

Chicago style: Thursday, November 12, 2022, was a normal day. {{CMS 2017, 9.31, 6.38}}
AP style: Saturday, Nov. 12, 2022, was a normal day. {{AP 2022, months, years}}

Day Month Date

Chicago style: Saturday, November 12, was a normal day. {{CMS 2017, 6.38}}
AP style: Saturday, Nov. 12, was a normal day {{AP 2022, months, days of the week}}

Month Date

Chicago style: Please call before October 1 to reserve your room. November 12 was a normal day. {{CMS 2017, 9.31, 6.38}}
AP style: Please call before Oct. 1 to reserve your room. Nov. 12 was a normal day. (Note, AP spells out the month unless a specific date is mentioned. So it’s Oct. 1 and Saturday, Oct. 1, and Oct. 1, 2022, but October 2022 and “in October.”) {{AP 2022, months, years}}

Month Year

November 2016 was a strange month. {{CMS 2017, 6.38; AP 2022, months}}
Note: Preferred over November of 2016.

Regarding “of,” CMS 17 includes the example “April 2021” in 6.38 and does not use “of” after the month anywhere on their site. And 5.250 (written by Bryan A. Garner) under “of” says, “Prefer June 2015 over June of 2015.” It is also covered in Bryan A. Garner, Garner’s Modern English Usage, 4th ed., p. 246, which states, “February 2010 is better than February of 2010.” (A big thank you to Glenn Gladfelder in July 2022 for noticing that “of” is not wrong according to CMS—it’s just less preferred. He is correct! I have updated this text to reflect that.)
AP only covers it in their “Ask the Editor” section here, where they say, “No of. Just March 2020. We may not have an explicit entry on that point, but an example is below (January 2016).”

Day only

They left on Tuesday for their vacation. {{CMS 2017, 8.88; AP 2022, days of the week}}

Month only

November was a strange month. {{CMS 2017, 6.38, 8.88; AP 2022, months}}
also: early November, mid-November, late November {{CMS 2017, 7.89 “mid,” AP 2022, mid-}}

Date only

Chicago style: His birthday is on the seventh. His birthday is on the twenty-first. We will be there the first through fifteenth of next month. {{CMS 2017, 9.31}}
AP style: His birthday is on the seventh. Her birthday is on the 21st. We will be there the first through 15th of next month.
{{AP does not have an entry for this, but their “Ask the Editor” advises us to follow their ordinal number rules in this situation: spell out first through ninth, and use numerals for 10th and above. This is the only exception to not using st, nd, rd, or th on the date. APStylebook.com, search “ordinal date” and read the “Ask the Editor” answers.}}

YEAR Only

CMS and AP agree, use numerals usually. But if the year is at the beginning of a sentence, CMS spells it out but AP does not.
Both agree: The transition will take place in 2025.
But CMS beginning of sentence: Twenty twenty was a year like no other. or Two thousand twenty was a year like no other. (CMS recommends rewording in these situations to avoid having the year begin the sentence.) {{CMS 2017, 9.29}}
AP beginning of sentence: 2020 was a year like no other. (AP usually spells out numbers at the beginning of a sentence, except for years.) {{AP 2022, years}}

Holiday with Year

Thanksgiving Day 2019 was chaotic for our family. (See also my “Holidays and Notable Days” post.) {{CMS 2017, 6.38, 8.89; AP 2022, holidays and holy days, AP Ask the Editor, “Christmas Day 1929,” https://apstylebook.com/ask_the_editors/15739.}}

Season with Year

Lowercase seasons.
In fall 2008 I took my first class. {{CMS 2017, 8.88; AP 2022, seasons}}

Exception: With CMS, in journal citations, capitalize the season. {{CMS 2017, 15.9}}
1. First Last, “Article Title,” Journal 36, no. 7 (Fall 2000): 25–31.

Dates as adjectives

Even if a date is used as an adjective, the comma rules don’t change.{{CMS 2017, 6.38; AP 2022, months, Ask-the-Editor}}

Chicago style: The February 10, 2022, concert was a success.
                        The Thursday, February 10, concert was a success.
                        The February 10 concert was a success.
AP style: The Feb. 10, 2022, concert was a success.
                The Thursday, Feb. 10, concert was a success.
                The Feb. 10 concert was a success.
Both: The February 2022 concert was a success.

Date with “on”

Chicago style: not specifically covered
AP style: Omit “on” with a date if it makes sense without it: The event will take place Nov. 16. (not “~~on Nov. 16~~” if possible) {{AP 2022, “on”}}

Abbreviations for Months and Days

In regular running text:

Chicago style: Do not abbreviate months or days of the week within regular text. {{CMS 2017, 9.31, 6.38}}

AP style: Spell out months with 5 letters or fewer within regular text. {{AP 2022, months}}

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
Days of the week: do not abbreviate.

In tabular material (lists, charts, tables, etc.):

Chicago style—There are three abbreviation systems listed in CMS. This is the first, and it is their preferred method. Spell out months with 4 letters or fewer. {{CMS 2017, 10.39}}

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. (tabular only)
Sun. Mon. Tues. Wed. Thurs. Fri. Sat. (tabular only)
Again, these abbreviations are OK for tabular matter only, such as lists, charts, and tables. DO NOT ABBREVIATE in regular running text.

AP style—In tabular material only (lists, charts, etc.), use these 3-letter forms with no period: {{AP 2022, months}}

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec (tabular only)
Sun Mon Tue Wed Thu Fri Sat (tabular only)
Again, these abbreviations are OK for tabular matter only, such as lists, charts, and tables. Within text, see above under “In regular running text.”

Abbreviated years

Chicago and AP styles: the class of ’16 {{CMS 2017, 9.30; AP 2022, numerals: dates, years and decades}}

Chicago style specifies this is for informal use only, and the full year is preferred formally: the class of 2016. {{CMS 2017, 9.30}}

Make sure to use an apostrophe/right single quote ’, not a left single quote ‘. Your software will assume you want a left single quote at the beginning of a word, so you have to go back and change it manually!

BC and AD

Chicago style: They invaded in 36 BC and AD 400. {{CMS 2017, 9.34, 10.38}}
AP style: They invaded in 36 B.C. and A.D. 400. {{AP 2022, B.C., A.D.}}

Note, AD goes before the year, but the other abbreviations go after the year. AP uses periods, Chicago does not.

BC and AD – “before Christ” and “anno Domini” (in the year of our Lord)
BCE and CE – “before the Common Era” and “of the Common Era”
(There are also others used for different purposes.)

Year Spans / Inclusive Years

If “from” or “between” precedes it, never use an en dash

from 2002 to 2004
between 2010 and 2015

Otherwise, you may use an en dash, which means “up to and including.”

Usually repeat only two digits.

in 1998–99
the 2005–10 crisis
in 2010-12
the winter of 2014–15

But repeat only one digit if it’s 01-09

in 2005–6

Repeat all digits if the century changes

in 1998–2002
in 1990–2010

Repeat all digits if one is an “00”

the years 2000–2001 were . . .
in 2000–2020

Repeat all digits if BC or BCE is used

300–200 BC

Other situations

To indicate the end of one year leading into the next, you may use a slash if you wish. Or just use an en dash or spell out in words.

the winter of 2015/16
the 1998/99 school year

For a book or chapter title, it’s best to repeat all digits. (If you’re just citing a book title, cite however it appears on the book.)

Diaries from the Trails, 1850–1855

You may also use an en dash when months and dates are included if context allows, as long as “from” or “between” does not precede it.

I will be gone December 2016–February 2017. but gone from December 2016 to February 2017.
The articles appeared January 29–March 20.
The May 14, 1966–January 10, 1967 issues are missing.

Centuries

Chicago style: the seventeen hundreds {{CMS 2017, 9.32}}
AP style: the 1700s {{AP 2022, years}}

Chicago style: the twentieth century {{CMS 2017, 9.32}}
AP style: the 20th century, but spell out centuries less than 10—the ninth century {{AP 2022, century, historical periods and events}}

Chicago notes regarding using centuries as adjectives

the style was very twentieth century but twentieth-century art {{CMS 7.89: century}}
the twenty-first century but twenty-first-century movies {{CMS 7.89: century}}

early twentieth-century fashion but fashion from the early twentieth century {{CMS 7.89: century}}
mid-twentieth-century fashion but fashion from the mid-twentieth century {{CMS 7.89: century, and prefixes—mid}}
late twentieth-century fashion but fashion from the late twentieth century {{CMS 7.89: century}}

the midcentury home {{CMS 7.89: prefixes—mid}}

Decades

Chicago and AP styles: the 1990s {{CMS 2017, 9.33; AP 2022, decades}}

Abbreviated decades:
Chicago style: the nineties (or the ’90s for less formal usage) {{CMS 2017, 9.33}}
AP style: the ’90s {{AP 2022, decades}}

(Make sure to use an apostrophe/right single quote ’, not a left single quote ‘. Your software will assume you want a left single quote at the beginning of a word, so you have to go back and change it manually!)

Chicago and AP styles: the early 1990s, the mid-1990s, the late 1990s {{CMS 2017, 7.89: prefixes—mid; AP 2022, mid-}}
Chicago style also accepts the early nineties, the midnineties or mid-1990s, the late nineties {{CMS 2017, 9.33; 7.89 prefixes: mid}}

1980s-style fashion and dressed 1980s-style {{CMS 2017, 7.89; AP: not specifically covered but this follows their style}}

Traditional decade names:
Chicago style: the Gay Nineties, the Roaring Twenties {{CMS 2017, 8.73}}
AP style: the Gay ’90s, the Roaring ’20s {{AP 2022, decades}}

The first two decades of a century must be treated differently.

1900–1909

the first decade of the twentieth century {{CMS 2017, 9.33}}
the years 1900–09 {{CMS 2017, 9.33}}
Do not use “~~the 1900s~~” to refer to just the first decade, because it could also refer to the whole century. {{CMS 2017, 9.33}}
AP style does not have an official ruling. Use whatever is clearest.

Note: Some experts consider a decade to begin in 1901, 2001, etc., but most readers consider a decade to begin in 1900, 2000, etc. Go with author/subject preference on this.

1910–19

“the 1910s” isn’t great, but can be used if necessary. {{CMS 2017, 9.33}}
the second decade of the twentieth century {{CMS 2017, 9.33}}
the years 2010–19 {{CMS 2017, 9.33}}
do not use “~~the teens~~.” {{CMS 2017, 9.33}}
AP style does not have an official ruling, but they do not like “the teens” either. Use whatever is clearest. {{AP Ask the Editor, search “1910s” and “the teens”}}

2000–09

the ’00s {{CMS Shop Talk, 2019, https://cmosshoptalk.com/2019/09/10/decades-cmos-9-33/}}
the first decade of the twenty-first century {{CMS 2017, 9.33}}
the years 2000–09 {{CMS 2017, 9.33}}
do not use “~~the 2000s~~” to refer to the decade only {{CMS 2017, 9.33}}
AP style does not have an official ruling, but tends to use “the 2000s” {{AP Ask the Editor, search “2000s” and “00s”}}

2010–19

the 2010s {{CMS Shop Talk, 2019, https://cmosshoptalk.com/2019/09/10/decades-cmos-9-33/}}
the ’10s {{CMS Shop Talk, 2019, https://cmosshoptalk.com/2019/09/10/decades-cmos-9-33/}}
the second decade of the twenty-first century {{CMS 2017, 9.33}}
the years 2010–19 {{CMS 2017, 9.33}}
do not use “~~the teens~~” {{CMS 2017, 9.33}}
AP style does not have an official ruling, but tends to use “the 2010s” {{AP Ask the Editor, search “2010s”}}

I tend to go with author preference on this. If the author has “the 2000s” and it’s clear they are talking about the decade, I feel it’s unnecessary to change. Check for clarity and adjust as needed.

The following is covered in Chicago Manual of Style only:

Introductory Adverbial Phrases with Dates

Per Chicago style. If the intro phrase is short, no comma is necessary unless it’s confusing without it. But you can still use a comma if you like. CMS approves both styles. They are not clear on exactly what “short” means, so use your judgment and make sure it’s clear. The shorter the phrase is, the less likely it will require a comma.

In 2015 I bought a new computer. (or In 2015, I . . . )
Before 1862 the troops were unorganized. (or Before 1862, the . . . )
On October 25 she won the lottery. (or On October 25, she . . . )
After January her health improved. (or After January, her . . . )

BUT

In 2003, students received new textbooks. (because “2003 students” could be confusing.)
In May, Jane sold her house. (because “May Jane” is confusing.)
After Monday, rush hour will be a nightmare. (Without the comma, the sentence is ambiguous.)

Note, you still need a comma before and after the year in month-day-year format, because you always do.

On October 25, 2015, he joined the company.

Note, this rule applies to all introductory adverbial phrases, not just the ones with dates.

After the divorce she left the city. (or After the divorce, she . . . )

but Before leaving, Jane said goodbye. (because “Before leaving Jane” is confusing.)

Lengths of time

a decades-old fight but the fight was decades old
a centuries-old document but the document was centuries old
a ten-year plan but the plan was for ten years
a six-month wait but the wait was six months
a three-day weekend or a three-day-long weekend but the weekend was three days long

Alternate and informal styles

Alternate (chiefly British) style: day-month-year style

Standard in British English. Can be used in US English, but primarily for works with many full dates. Most US readers are used to the other style, so it can be distracting.

No commas needed.

The events of 5 April 2012 and 17 June 2015 were oddly similar.

Slashes, dashes, and periods

Avoid these in formal publications because US puts month first, but many other countries put day first (e.g., Canada, UK).

3/26/14 3-26-14 3.26.14 (informal use only)

ISO standard date format

If an all-numeral form is needed, use the ISO standard date format. Not recommended for formal prose, but allows dates to be sorted properly in lists and spreadsheets.

2014-03-26 (for lists or informal use only)

Sources:

Chicago Manual of Style, 17th Edition, 5.83, 6.38, 6.69, 6.78, 6.107–8; 7.89 under “numbers, spelled out,” “century,” “old,” “style,” “prefixes: mid”; 8.72–73, 8.88–89, 9.29–36, 9.64, 10.38–40. (Not used here but relevant: 6.68, 6.79, 9.34)
Associated Press Stylebook 2020–22, months, years, dates, A.D., B.C., century, decades

Chicago style dictionary is Merriam Webster’s Collegiate Dictionary, Eleventh Edition (MW11)
AP style dictionary is Webster’s New World Collegiate Dictionary, Fifth Edition (WNW5)