A manual for writers of research papers, theses, and dissertations, Ninth edition - Kate L. Turabian 2018

Date systems
Numbers
Style

23.3.1 Month, Day, and Year

Spell out the names of months when they occur in text, whether alone or in dates. Express days and years in numerals, and avoid using them at the beginning of a sentence, where they would have to be spelled out (see 23.1.2.1). Do not abbreviate references to the year (“the great flood of ’05”). For abbreviations acceptable in tables, figures, and citations, see 24.4.2.

Every September, we recall the events of 2001.

but not

Two thousand one was a memorable year.

For full references to dates, give the month, the day (followed by a comma), and the year, in accordance with US practices. If you omit the day, omit the comma. Also omit the comma for dates given with seasons instead of months; do not capitalize the names of seasons (see 22.1.2). If you are quoting material that uses British-style dates (15 March 2007), do not alter them.

President John F. Kennedy was assassinated on November 22, 1963.

By March 1865, the war was nearly over.

The research was conducted over several weeks in spring 2006.

Note that within complete dates, days are generally not given as ordinals—that is, the numerals are not followed by st, nd, rd, or th. Use these endings only with spelled-out numbers when you specify the day without the month or year.

The date chosen for the raid was the twenty-ninth.

but not

The events occurred on June 11th, 1968.

23.3.2 Decades, Centuries, and Eras

In general, refer to decades using numerals, including the century (see 23.2.1 for plurals). If the century is clear, you may spell out the name of the decade; do not abbreviate numerals (“the ’90s”). The first two decades of any century do not lend themselves to either style and, for clarity, should not be referred to in a shortened form.

The 1920s brought unheralded financial prosperity.

During the fifties, the Cold War dominated the headlines.

Many of these discoveries were announced during the first decade of the twenty-first century.

Refer to centuries using either numerals or lowercase spelled-out names (see 23.2.1 for plurals). If the century is spelled out and used as an adjective preceding a noun that it modifies, as in the second example, include a hyphen; otherwise do not (see 20.3.2).

The Ottoman Empire reached its apex in the 1600s.

She teaches nineteenth-century novels but would rather teach poetry from the twentieth century.

The most common designations for eras use the abbreviations BC (“before Christ”) and AD (anno Domini, “in the year of the Lord”). Some disciplines use different designations, such as BCE and CE (see 24.4.3). AD precedes the year number; the other designations follow it. For inclusive numbers with eras, see 23.2.4.

Solomon’s Temple was destroyed by the Babylonians in 586 BC and again by the Romans in AD 70.