Punctuation and Formatting for Dates

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Chicago Manual of Style 16 (books) and AP Stylebook 2015 (media) agree unless noted.

Date Formatting

Chicago and AP agree, do not use st, nd, rd, or th on date, even though you pronounce it.
With full date, use commas before and after year.

Chicago style: The concert will be February 10, 2016, at Carson Music Hall.
AP style: The concert will be Feb. 10, 2016, at Carson Music Hall.

Chicago style: Thursday, November 12, 2015, was a normal day.
AP style: Thursday, Nov. 12, 2015, was a normal day.

Chicago style: Please call before October 1 to reserve your room.
AP style: Please call before Oct. 1 to reserve your room.

November 12 was a normal day.
Thursday, November 12,
was a normal day.*

November 2016 was a strange month.
Note: not November of 2016.**

Christmas Day 2016 was chaotic for our family.*

November was a strange month.
also: early November, mid-November, late November

They left on Tuesday for their vacation.

Dates as adjectives

Chicago style—You still have to use a comma after the year, which is awkward, so Chicago recommends rewriting the sentence to avoid this problem.

The February 27, 2016, concert was a success.
PREFERABLY REWRITE: The concert on February 10, 2016, was a success.

But no commas needed with

the February 27 concert
the February 2016 concert

AP style—Use a comma:
The Feb. 27, 2016, concert was a success.

Date alone

Chicago style—Spell out all. Hyphenate if two words:
We arrived on the seventh before dinner.
You have until the twenty-fifth to decide.

AP style—Spell out first through ninth, ordinal numerals for others
the first of the month, the 15th of the month, the first through 15th of the month

AP note: Omit “on” with a date if it makes sense without it:

The event will take place Nov. 16. (not “on Nov. 16“)

* This one is not specifically mentioned by AP. CMS 16 did not mention it, but CMS 17 includes it in 6.38.

** This is not specifically mentioned by AP, but they follow it. CMS 16 did not mention it, but CMS 17 includes it in 6.38. It is also covered in Bryan A. Garner’s Modern American Usage, p. 226, which states, “February 2010 is better than February of 2010. Stylebooks have long agreed that no comma should appear between the month and the year.”

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Abbreviations for Months and Days

In regular, running text:

Chicago style: Do not abbreviate months or days of the week within regular text.

AP style: Spell out months with 5 letters or fewer within regular text.

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 is their preferred method. Spell out months with 4 letters or fewer.

Jan.  Feb.  Mar.  Apr.  May  June  July  Aug.  Sept.  Oct.  Nov.  Dec. (tabular only)
Sun.  Mon.  Tues.  Wed.  Thurs.  Fri.  Sat. (tabular only)

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

Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec  (tabular only)
Sun  Mon  Tue  Wed  Thu  Fri  Sat (tabular only)

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Holidays and seasons

Capitalize holiday names, lowercase seasons. No commas.

On Independence Day 2014 we held a party.
In fall 2008 I took my first class.

(Sources: CMS 8.87, 8.88; AP seasons, holidays)

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Year Alone

Use numerals usually

We’ve come a long way since our 1996 graduation.

BUT, if it’s at the beginning of a sentence:

Chicago style—Spell out a year at the beginning of a sentence. They recognize that this looks strange, and recommend rewording the sentence to avoid it.

Instead of: 2016 is finally over.
Use: Twenty sixteen is finally over. or Two thousand sixteen is finally over.
Or reword: I can’t believe 2016 is finally over.
Also note: Nineteen sixty-three was the year it happened.

AP style—You can begin a sentence with a numeral year. (But any other numbers at the beginning of a sentence must be spelled out.)

2016 is finally over.

Abbreviated years

Chicago and AP styles: the class of ’16

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

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!

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BC and AD

Chicago style: They invaded in 36 BC and AD 400.
AP style: They invaded in 36 B.C. and A.D. 400.

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.)

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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.

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Centuries

Chicago style: the seventeen hundreds
AP style: the 1700s

Chicago style: the twentieth century
AP style: the 20th century, but spell out centuries less than 10—the ninth century

Chicago notes on using centuries as adjectives

the style was very twentieth century  but  twentieth-century art
the twenty-first century  but  twenty-first-century movies

early twentieth-century fashion  but  fashion from the early twentieth century
mid-twentieth-century fashion  but  fashion from the mid-twentieth century
late twentieth-century fashion  but  fashion from the late twentieth century

the midcentury home

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Decades

Chicago and AP styles: the 1990s

Abbreviated decades:
Chicago style: the nineties (or the ’90s for informal usage only)
AP style: the ’90s

(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
Chicago style also accepts the early nineties, the midnineties, the late nineties (7.85 p.383)

Chicago style: 1980s-style fashion  and  dressed 1980s-style
Chicago style: Traditional decade names—the Gay Nineties, the Roaring Twenties

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

1900–1909

the first decade of the twentieth century
the years 1900–09
Do not use “the 1900s” to refer to just the first decade, because it could also refer to the whole century.

Note: Some experts consider a decade to begin in ‘_1 (not ‘_0). Go with your author’s preference on this.

1910–19

“the 1910s” isn’t great, but can be used if necessary.
the second decade of the twentieth century
the years 2010–19
do not use “the teens.”

2000–09

the first decade of the twenty-first century
the years 2000–09
do not use “the 2000s” to refer to the decade only

2010–19

“the 2010s” isn’t great, but can be used if necessary.
the second decade of the twenty-first century
the years 2010–19
do not use “the teens

Even CMS’s popular Q&A section online does not have a better answer for this. “I liked the eighties, loved the nineties, and hated the first decade of the twenty-first century.” Just doesn’t have a good ring to it, does it?

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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.)

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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

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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)

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Sources:

Chicago Manual of Style, 16th Edition, 5.82, 6.45, 6.69, 6.78, 6.105–6; 7.85 p. 376, 379, 381–3; 8.70, 8.72, 8.87–88, 9.30–37, 9.63, 10.40–41. (Not used here but relevant: 6.66, 6.79, 9.35)
Associated Press Stylebook 2015.

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)