When to Hyphenate Numbers with Units of Time

Do you have a minute? The following three guidelines explain when to hyphenate numbers with units of time such as hours, days, months, years, and centuries.

Before we begin, here are a couple of brief words on multi-word descriptions: multi-word descriptions, such as three-hour, that come before nouns are called compound adjectives, compound modifiers, and phrasal adjectives.

1. Hyphenate Numbers with Units of Time before Nouns

Hyphenate spelled-out numbers with units of time that appear directly before the nouns they are describing.¹

The skipper and his crew started a three-hour tour of the Pacific Ocean.

The couple signed a thirty-year mortgage on their dream home.

Icelandic historian Snorri Sturluson wrote the thirteen-century book Prose Edda.

2. Hyphenate Numerals with Units of Time before Nouns

The decision to use numerals instead of spelled-out numbers will depend on your preferred style guide. Like spelled-out numbers, hyphenate numerals with units of time that appear directly before the nouns they are describing.²

A 45-day drought damaged the soybean crops.

The student bought a new 12-month desk calendar.

The company debuted its 30-second commercial during the football game.

3. Don’t Hyphenate Standalone Numbers with Units of Time

Spelled-out numbers or numerals with units of time that don’t appear directly before a noun—in other words, those that aren’t acting as compound adjectives—don’t need to be hyphenated.

However, double-digit numbers (e.g., forty-one, ninety-nine) should maintain their normal internal hyphenation.

The electric light bulb was invented in the nineteenth century.

All the kids were excited that school started in twenty-one days.

The road trip took sixteen hours, three tanks of gas, and five cups of coffee.

Do you have a bit of free time on your hands? Check out my related posts for more information on writing about time:

How to Write Decades as Words and Numerals

How to Write Centuries as Words and Numerals

When Should You Capitalize Historical Time Periods?

References

1. The Associated Press Stylebook 2019 (New York: Associated Press, 2019), 204–06; The Chicago Manual of Style, 17th ed. (Chicago: University of Chicago Press, 2017), 447; Publication Manual of the American Psychological Association, 7th ed. (Washington, DC: American Psychological Association, 2020), 6.12.
2. The Associated Press Stylebook 2019 (New York: Associated Press, 2019), 204–06; The Chicago Manual of Style, 17th ed. (Chicago: University of Chicago Press, 2017), 447; Publication Manual of the American Psychological Association, 7th ed. (Washington, DC: American Psychological Association, 2020), 6.12.