You can show time by using either the 24-hour or 12-hour clock.

  • The 24-hour clock is the preferred format for international audiences and the format used in most computer systems, so use the 24-hour clock when possible.

  • If the technology or interface that you’re documenting shows or uses the 12-hour clock, then be consistent with the interface and use the 12-hour clock.

24-hour clock#

When you use the 24-hour clock to show time, use the following guidelines:

  • Use a colon to separate the hours, minutes, and seconds.

  • Show the hours, minutes, and seconds with two digits each, even if the leading digit is 0.

  • If you need to show a time zone, use Coordinated Universal Time (UTC), and indicate the time-zone offset from UTC.




18:00:00 to 20:30:00

10:30:00 (UTC -6) (refers to CT)

12:00:00 (noon)

00:00:00 (midnight)

12-hour clock#

When you use the 12-hour clock to show time, use the following guidelines:

  • Use a colon to separate the hours and minutes. If the minutes are 00, you don’t need to show them unless you’re showing a span of time that includes a time with minutes.

  • Use uppercase letters for abbreviations of ante meridiem (AM) and post meridiem (PM). Separate these abbreviations from the time with a space. Do not use periods in the abbreviations.

  • When specifying time zones, show both the spelled-out name and the abbreviation. Show the name in lowercase letters; use uppercase letters and no periods for the abbreviation.

  • Avoid references to standard and daylight saving time because the appropriate designation changes frequently. However, if you need to include such a reference, insert S (for standard) or D (for daylight) as the second character in the abbreviation.

  • When referring to 12 AM, use 12 midnight or just midnight. When referring to 12 PM, use 12 noon or just noon.


10:29 AM

6 PM

6:00 PM to 8:30 PM

10:30 AM Central Time (CT)

1:30 PM Central Standard Time (CST)