A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Decades, centuries, and eras
Numbers
Part III. Style 20 spelling

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

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, use 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.