AP Style states to capitalize the full name of the time in force within a particular zone. For example,
- Eastern Standard Time
- Eastern Daylight Time
- Central Standard Time
Lowercase all but the region in short forms. For example,
- the Eastern time zone
- Eastern time
- Mountain time
See AP Style Time of Day for guidelines on when to use clock time in a story.
Spell out “time zone” in references not accompanied by a clock reading. For example,
- Chicago is in the Central time zone.
The abbreviations EST, CDT, etc., are acceptable on first reference for zones used within the continental United States, Canada, and Mexico only if the abbreviation is linked with a clock reading. For example,
- noon EST
- 9 a.m. PST
Do not set off the abbreviations with commas.
Spell out all references to time zones not used within the contiguous United States. For example,
- When it is noon EDT, it is 1 p.m. Atlantic Standard Time and 8 a.m. Alaska Standard Time.
There is one exception to the spelled-out form. Greenwich Mean Time may be abbreviated as GMT on second reference if used with a clock reading.
See AP Style GMT