Format for spans of time

Question:

What is the correct format for presenting a span of time such as 10:00–11:00 a.m.?

Answer:

There does not seem to be a set format for presenting a span of time in English; however, in "Numbers in a Sequence," The Gregg Reference Manual outlines several rules that can be applied to spans of time.

1. Use an en dash (a short dash) to link two figures that represent a continuous sequence. Do not put a space on either side of the en dash.

   Example:

   • Lunch — 12:00 p.m.-1:00 p.m.
   • Plenary session — 1:00 p.m.-2:30 p.m.
   • Closing — 4:00 p.m.-4:15 p.m.
2. Do not use the en dash if the sequence is introduced by the preposition from or between.

   Example:

   • The party is from 9 p.m. to 2 a.m.
     NOT … from 9 p.m.-2 a.m.
   • We will meet between 3:30 p.m. and 3:45 p.m.
     NOT … between 3:30 p.m.-3:45 p.m.

Sometimes the first a.m. or p.m. is omitted when the span of time occurs in the same portion of the day, morning or afternoon. However, in formal writing, you need to repeat the abbreviation.

Example:

• The office is closed from 1 p.m. to 2 p.m. for lunch.
  NOT … from 1 to 2 p.m. for lunch.