When shortening a year to its last two digits, use an apostrophe.

The two most common pairs of abbreviations for eras are B.C.(before Christ) and A.D. (anno Domini, in the year of our lord); and B.C.E. (before the common era) and C.E. (common era). SparkNotes puts these abbreviations in small caps and places periods between the letters.

You may spell out decades or not, according to your preference—just be consistent in your writing.

When referring to decades, use s alone, not an apostrophe and an s.

Spell out centuries; do not capitalize them.

If you’re naming a month, day, and year, in that order, include commas after the day and the year.

If you’re being pretentious and naming a day, month, and year, in that order, no commas are necessary.

If you’re naming a month and a year, no commas are necessary.

Capitalize days of the week, months, and holidays. Do not capitalize seasons.

Don’t use slashes when noting dates in formal writing.

If you’re mentioning a date without attaching it to a month, spell it out.