What does AM and PM mean?

AM stands for Ante Meridiem (Latin: "before midday") and PM stands for Post Meridiem ("after midday"). AM covers midnight to noon (12:00 AM to 11:59 AM). PM covers noon to midnight (12:00 PM to 11:59 PM). The confusing part: 12:00 AM is midnight and 12:00 PM is noon — not the other way around.

EST stands for Eastern Standard Time. It is UTC-5 and is used on the US East Coast (New York, Miami, Atlanta, Boston) during winter — from the first Sunday of November to the second Sunday of March. In summer, the East Coast switches to EDT (Eastern Daylight Time, UTC-4).

CST stands for Central Standard Time. It is UTC-6 and is used in the central US (Chicago, Dallas, Houston, Minneapolis) during winter. In summer, Central Time switches to CDT (Central Daylight Time, UTC-5). CST is 1 hour behind EST and 2 hours ahead of PST.

PST stands for Pacific Standard Time. It is UTC-8 and is used on the US West Coast (Los Angeles, Seattle, San Francisco) during winter. In summer, Pacific Time switches to PDT (Pacific Daylight Time, UTC-7). PST is 3 hours behind EST.

CDT stands for Central Daylight Time. It is UTC-5 and is the summer version of Central Time, active from the second Sunday of March to the first Sunday of November. CDT is 1 hour ahead of CST (the winter version). During CDT, Chicago is on the same UTC offset as New York's winter EST.

MDT stands for Mountain Daylight Time. It is UTC-6 and is the summer version of Mountain Time, active from the second Sunday of March to the first Sunday of November. MDT is 1 hour ahead of MST (the winter version). Note: Arizona does not observe MDT — it stays on MST year-round.

EDT stands for Eastern Daylight Time. It is UTC-4 and is the summer version of Eastern Time, active from the second Sunday of March to the first Sunday of November. EDT is 1 hour ahead of EST. When New York is on EDT, it is 4 hours behind London (BST).

PDT stands for Pacific Daylight Time. It is UTC-7 and is the summer version of Pacific Time, active from the second Sunday of March to the first Sunday of November. PDT is 1 hour ahead of PST. The gap between PDT and EDT is still 3 hours — same as PST vs EST.

What is the difference between EST and ET?

EST (Eastern Standard Time) is the specific winter version of Eastern Time (UTC-5). ET (Eastern Time) is the general, season-neutral label that works year-round. Use EST only in winter (November–March), EDT only in summer (March–November), and ET whenever you want to be safe regardless of season.

UTC stands for Coordinated Universal Time. It is the modern international time standard maintained by atomic clocks. UTC is the reference point from which all time zones are calculated — for example, EST is UTC-5 (5 hours behind UTC). UTC never observes Daylight Saving Time.

GMT stands for Greenwich Mean Time. It is the historical time standard based on solar time at the Royal Observatory in Greenwich, London. GMT is UTC+0. For everyday purposes, GMT and UTC are identical. The UK uses GMT in winter and BST (British Summer Time, UTC+1) in summer.

DST stands for Daylight Saving Time. It is the practice of advancing clocks by 1 hour in spring ("spring forward") and setting them back in fall ("fall back"). In the US, DST runs from the second Sunday of March to the first Sunday of November. Not all countries observe DST — India, Japan, China, and most of Africa do not.

Every abbreviation explained simply

What Do Time Abbreviations Mean?

AM/PM, EST, CST, PST, EDT, CDT, MDT, PDT, GMT, UTC — every time abbreviation decoded with plain-English explanations, UTC offsets, and example cities.

AM/PMESTCSTPSTEDTCDTMDTPDTGMTUTCISTJST

What Does AM/PM Mean?

AMAnte Meridiem

Latin: "before midday"

12:00 AM (midnight) → 11:59 AM

AM covers the first half of the day — from midnight through to just before noon. 12:00 AM is midnight (the start of the day), and 11:59 AM is one minute before noon.

12:00 AM — Midnight
1:00 AM — 1 hour after midnight
6:00 AM — Early morning
11:59 AM — Just before noon

PMPost Meridiem

Latin: "after midday"

12:00 PM (noon) → 11:59 PM

PM covers the second half of the day — from noon through to just before midnight. 12:00 PM is noon, and 11:59 PM is one minute before midnight.

12:00 PM — Noon (midday)
1:00 PM — 1 hour after noon
6:00 PM — Evening
11:59 PM — Just before midnight

The 12 AM / 12 PM Confusion

At exactly 12:00, you are at the meridiem itself — neither before nor after it. By convention: 12:00 AM = midnight and 12:00 PM = noon. This trips up almost everyone.

12:00 AM

Midnight

Start of day

12:01 AM

Just after midnight

12:00 PM

Noon

Middle of day

12:01 PM

Just after noon

Tip: To avoid confusion, say "noon" instead of 12 PM and "midnight" instead of 12 AM. Or use 24-hour time: noon = 12:00, midnight = 00:00.

US Time Zone Abbreviations

The US has 4 mainland time zones. Each has two abbreviations — one for winter (standard time) and one for summer (daylight time).

EST

UTC-5

Eastern Standard Time

Winter (Nov–Mar)

US East Coast

Cities: New York, Miami, Atlanta, Boston

CST

UTC-6

Central Standard Time

Winter (Nov–Mar)

US Midwest & South

Cities: Chicago, Dallas, Houston, Minneapolis

MST

UTC-7

Mountain Standard Time

Winter (Nov–Mar)

US Mountain West

Cities: Denver, Phoenix, Salt Lake City

PST

UTC-8

Pacific Standard Time

Winter (Nov–Mar)

US West Coast

Cities: Los Angeles, Seattle, San Francisco

AKST

UTC-9

Alaska Standard Time

Winter (Nov–Mar)

Alaska

Cities: Anchorage, Fairbanks

HST

UTC-10

Hawaii Standard Time

Year-round (no DST)

Hawaii

Cities: Honolulu, Hilo

Note: Standard time abbreviations (EST, CST, MST, PST) are only technically correct in winter. In summer, use the daylight versions (EDT, CDT, MDT, PDT). If you want a term that works year-round, use the general labels: ET, CT, MT, PT.

Standard vs Daylight: What Changes?

Each US time zone has two names — one for winter, one for summer. The only thing that changes is the UTC offset (by 1 hour).

Zone	Winter (Standard)	UTC	Summer (Daylight)	UTC	General Label
Eastern	EST	UTC-5	EDT	UTC-4	ET
Central	CST	UTC-6	CDT	UTC-5	CT
Mountain	MST	UTC-7	MDT	UTC-6	MT
Pacific	PST	UTC-8	PDT	UTC-7	PT
Alaska	AKST	UTC-9	AKDT	UTC-8	AKT
Hawaii	HST	UTC-10	— (no DST)	—	HT

DST switch dates 2026: Spring forward March 8 · Fall back November 1

CDT and MDT Explained

CDT

Central Daylight Time · UTC-5

Active: March 8 – November 1, 2026

CDT is Central Time during Daylight Saving Time (summer).
CDT = UTC-5. CST (winter) = UTC-6. CDT is 1 hour ahead of CST.
CDT is 1 hour behind EDT (Eastern Daylight Time).
CDT is 2 hours ahead of PDT (Pacific Daylight Time).
Cities: Chicago, Dallas, Houston, Minneapolis, New Orleans.
Example: 9:00 AM CDT = 10:00 AM EDT = 7:00 AM PDT.

MDT

Mountain Daylight Time · UTC-6

Active: March 8 – November 1, 2026

MDT is Mountain Time during Daylight Saving Time (summer).
MDT = UTC-6. MST (winter) = UTC-7. MDT is 1 hour ahead of MST.
MDT is 2 hours behind EDT and 1 hour behind CDT.
MDT is 1 hour ahead of PDT (Pacific Daylight Time).
Arizona does NOT observe MDT — it stays on MST year-round.
Cities: Denver, Salt Lake City, Boise (not Phoenix).

World Time Abbreviations

The most common international time zone abbreviations and what they mean.

Abbr	Full Name	UTC Offset	Region	Note
GMT	Greenwich Mean Time	UTC+0	UK (winter), West Africa, Iceland	The original world time standard. Never changes.
UTC	Coordinated Universal Time	UTC+0	Global standard	The modern atomic-clock-based standard. Identical to GMT for everyday use.
BST	British Summer Time	UTC+1	UK (summer)	UK's DST version of GMT. Active late March to late October.
CET	Central European Time	UTC+1	France, Germany, Italy, Spain (winter)	Most of continental Europe in winter.
CEST	Central European Summer Time	UTC+2	France, Germany, Italy, Spain (summer)	CET + 1 hour during DST. Active late March to late October.
EET	Eastern European Time	UTC+2	Greece, Romania, Bulgaria (winter)	Eastern Europe in winter.
MSK	Moscow Standard Time	UTC+3	Russia (Moscow)	Russia abolished DST in 2014. MSK is year-round.
IST	India Standard Time	UTC+5:30	India (all)	India uses a single time zone for the entire country. No DST.
CST	China Standard Time	UTC+8	China (all)	China uses one time zone for the entire country. No DST.
SGT	Singapore Time	UTC+8	Singapore	No DST. Same offset as China Standard Time.
JST	Japan Standard Time	UTC+9	Japan	Japan does not observe DST. JST is year-round.
KST	Korea Standard Time	UTC+9	South Korea	No DST. Same offset as Japan Standard Time.
AEST	Australian Eastern Standard Time	UTC+10	QLD, NSW, VIC, TAS, ACT (winter)	Queensland uses AEST year-round. Other states switch to AEDT in summer.
AEDT	Australian Eastern Daylight Time	UTC+11	NSW, VIC, TAS, ACT (summer)	Australian DST version. Active October to April.
NZST	New Zealand Standard Time	UTC+12	New Zealand (winter)	New Zealand observes DST, switching to NZDT (UTC+13) in summer.

General Time Zone Labels (ET, CT, MT, PT)

These two-letter labels work year-round — they don't specify whether it's standard or daylight time. Use them when you want to be safe regardless of season.

ETEastern Time

General label for the Eastern US zone. Works year-round — covers both EST and EDT.

CTCentral Time

General label for the Central US zone. Covers both CST and CDT.

MTMountain Time

General label for the Mountain US zone. Covers both MST and MDT.

PTPacific Time

General label for the Pacific US zone. Covers both PST and PDT.

AKTAlaska Time

General label for Alaska. Covers both AKST and AKDT.

HTHawaii Time

General label for Hawaii. No DST, so HT = HST year-round.