What Do Time Abbreviations Mean?
Time abbreviations can be confusing — especially when the same letters mean different things in different seasons. Here's a plain-English guide to every common time abbreviation.
View the full interactive guide →What Does AM/PM Mean?
- AM = Ante Meridiem (Latin: "before midday"). Covers 12:00 AM (midnight) to 11:59 AM.
- PM = Post Meridiem (Latin: "after midday"). Covers 12:00 PM (noon) to 11:59 PM.
- 12:00 AM = midnight (start of day). 12:00 PM = noon (middle of day).
- To avoid confusion, say "noon" instead of 12 PM and "midnight" instead of 12 AM.
What Does EST Mean?
EST = Eastern Standard Time = UTC-5. Used on the US East Coast (New York, Miami, Atlanta) in winter — from the first Sunday of November to the second Sunday of March. In summer, the East Coast switches to EDT (UTC-4).What Does CST Mean?
CST = Central Standard Time = UTC-6. Used in the central US (Chicago, Dallas, Houston) in winter. In summer, Central Time switches to CDT (UTC-5). CST is 1 hour behind EST.What Does PST Mean?
PST = Pacific Standard Time = UTC-8. Used on the US West Coast (Los Angeles, Seattle) in winter. In summer, Pacific Time switches to PDT (UTC-7). PST is 3 hours behind EST.What Does CDT Mean?
CDT = Central Daylight Time = UTC-5. The summer version of Central Time, active March–November. CDT is 1 hour ahead of CST. During CDT, Chicago is on the same UTC offset as New York's winter EST.What Does MDT Mean?
MDT = Mountain Daylight Time = UTC-6. The summer version of Mountain Time, active March–November. MDT is 1 hour ahead of MST. Arizona does NOT observe MDT — it stays on MST year-round.Standard vs Daylight: Quick Reference
| Zone | Winter | UTC | Summer | UTC |
|---|---|---|---|---|
| Eastern | EST | UTC-5 | EDT | UTC-4 |
| Central | CST | UTC-6 | CDT | UTC-5 |
| Mountain | MST | UTC-7 | MDT | UTC-6 |
| Pacific | PST | UTC-8 | PDT | UTC-7 |
General Labels (ET, CT, MT, PT)
These two-letter labels work year-round — they don't specify whether it's standard or daylight time. Use ET instead of EST/EDT, CT instead of CST/CDT, etc., when you want to be safe regardless of season.
