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 2018 (New York: Associated Press, 2018), 207–08; The Chicago Manual of Style, 17th ed. (Chicago: University of Chicago Press, 2017), 447; Publication Manual of the American Psychological Association, 6th ed. (Washington, DC: American Psychological Association), 98.
2. The Associated Press Stylebook 2018 (New York: Associated Press, 2018), 207–08; The Chicago Manual of Style, 17th ed. (Chicago: University of Chicago Press, 2017), 447; Publication Manual of the American Psychological Association, 6th ed. (Washington, DC: American Psychological Association), 98.