How to Write Time Zones
Summary
Indicate the time zone when not doing so would cause confusion. Abbreviations (e.g., GMT, EDT, IST) are generally used within a national context. Full forms of time zones may be capitalized or lowercased (eastern standard time or Eastern Standard Time). When lowercased, proper nouns should still be capitalized (central European time, Pacific standard time).
Abbreviations for time zones may be enclosed in parentheses or brackets, but don’t enclose them in commas (2:30 p.m. EST). Do use commas if the time zone is spelled out (2:30 p.m., Eastern Standard Time).
UTC offsets may be preferred in international publications (e.g., UTC-05:00 for EST).
When to show a time zone
Always specify the time zone when not specifying one would cause confusion. If the reader of your report, article, or other document is not local to a single time zone, specify the time zone for all clock times mentioned. Similarly, if the times mentioned span different time zones, make sure to clearly indicate which zone you’re referring to.
- Farley’s flight took off from O’Hare International Airport at 2:30 p.m. CST on June 23, 1963, before vanishing into thin air.
- It landed in Denver on June 23, 2013, at 4:43 a.m. MST.
- The online conference begins at 9:30 a.m. GMT on Monday.
However, if you and your readers are in the same time zone (for example, within the same physical office), or if the time zone is immaterial (like in a piece of creative writing), it is unnecessary information and should be omitted.
If you and the recipient of your communication are in different time zones, always specify the time zone to avoid confusion. Don’t hesitate to specify two or more time zones if that can help avoid confusion.
- Can we have a quick call at 12:30 p.m. CST (6:30 p.m. GMT)?
- At 4:30 p.m. IST (6:00 a.m. EST), the furnace exploded.
Time zone abbreviations vs. full forms
When indicated alongside a clock time, time zones are usually abbreviated. But when a clock reading isn’t mentioned, they are spelled out.
- The train arrives in Luzern at 10:45 a.m. CET.
butAll train timings shown are Central European Time.
- The meeting begins at 12:30 p.m. MT.
butAre the time frames listed Mountain Time or Eastern Standard Time?
When it is clear which time zone is being referred to, abbreviations are usually sufficient, especially within a national context. But note that these abbreviations are not internationally standardized and can cause confusion: for example, PST is both Pacific Standard Time and Philippine Standard Time. Similarly, IST can be Indian Standard Time, Irish Standard Time, or Israel Standard Time. If there is any chance your audience could be confused, spell out the time zone instead of using an abbreviation.
- The online class starts at 2:30 p.m., Philippine standard time.
- You can start logging in from 8:30 a.m., Irish standard time.
Use an abbreviation for a time zone only if you’re sure your audience will immediately recognize it; otherwise, use the full form.
- The program starts at noon PST.
in a national or local context
- The program starts at noon, Pacific Standard Time.
in an international context
Capitalization of time zones
When spelled out, time zones may be either capitalized or lowercased. Style manuals differ in their recommendations: The Chicago Manual of Style, followed in U.S. book publishing, recommends lowercasing them. In contrast, the AP Stylebook suggests capitalizing the full forms of time zones. Choose a style and follow it consistently across a document.
- Chicago: The first rocket was launched at 2:00 p.m., Indian standard time.
orAP: The first rocket was launched at 2:00 p.m., Indian Standard Time.
- Chicago: All timings listed are in Newfoundland standard time.
orAP: All timings listed are in Newfoundland Standard Time.
When the full forms of time zones are lowercased, proper nouns (and proper adjectives) should still be capitalized.
- Incorrect: When I said 2:30, I meant alaska standard time, not pacific standard time.
Correct: When I said 2:30, I meant Alaska standard time, not Pacific standard time.
Periods in abbreviations of time zones
Don’t insert periods after the letters of a time zone. Abbreviations with all capital letters generally don’t contain internal punctuation.
- Poor: When it’s 3 p.m. A.M.T., it’s 6 a.m. F.J.T.
Better: When it’s 3 p.m. AMT, it’s 6 a.m. FJT. - Poor: The sale starts at 9:30 a.m. E.D.T. tomorrow.
Better: The sale starts at 9:30 a.m. EDT tomorrow.
Commas around time zones
Don’t enclose the abbreviation for a time zone in commas. However, if you spell out a time zone accompanied by a clock time, enclose it in commas.
- The Durandians landed in Odesa at 9:17 a.m. EET.
butThe Durandians landed in Odesa at 9:17 a.m., Eastern European Standard Time.
- At 9:30 a.m. BST, it’s 6:00 p.m. ACST.
Of course, if the time zone appears in running text without an accompanying clock reading, don’t unnecessarily set it off with commas.
Parentheses or brackets around time zones
In running text, time zones may be enclosed in parentheses (or round brackets). This is optional: the Chicago Manual of Style recommends using parentheses; the AP Stylebook suggests omitting them.
- Correct: Chicago: The meteor shower began at 4:23 a.m. (JST).
Correct: AP: The meteor shower began at 4:23 a.m. JST.
Daylight saving time
Many regions and territories of the world set clocks forward and back in summer and winter. The letter D in a time-zone abbreviation usually indicates that the time zone is in daylight saving time (DST).
- We beat a cyclone to finally land at 3:33 a.m. ACDT.
Australian Central Daylight Time
- Join us at noon EDT for a talk on the latest mission to Mars.
Eastern Daylight Time
It’s daylight saving time, not daylight savings time.
GMT vs. UTC
Coordinated Universal Time (UTC), the primary global time standard, indicates the same time as GMT but has now superseded GMT in international use. In many international publications, particularly academic and scientific, UTC offsets are preferred over references to local time zones (EST, PST, etc.). Since abbreviations of time zones are not standardized across the globe, they can be confusing to an international audience. A UTC offset is shown as plus (+) or minus (-) the number of hours and minutes it is ahead of or behind UTC.
- Join us at noon UTC-04:00 to hear more about dolphins.
a time zone 4 hours behind UTC (e.g., EDT in the United States)
- Please call us between 9 a.m. and 6 p.m. UTC+05:30 on weekdays.
a time zone 5 hours and 30 minutes ahead of UTC (e.g., IST in India)
While writing or editing a document, check your client’s preferred house style to decide whether to use abbreviations for time zones or offsets from UTC.