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ADT/AST

Atlantic Daylight Time

UTC−3

12am

3am

6am

9am

12pm

3pm

6pm

9pm

Atlantic Time is on daylight saving time: ADT (UTC−3) until Sun, Nov 1.

Use AST all year (UTC−4)

MDT/MST

Mountain Daylight Time

UTC−6

3h behind

12am

3am

6am

9am

12pm

3pm

6pm

9pm

Mountain Time is on daylight saving time: MDT (UTC−6) until Sun, Nov 1.

Arizona stays on MST (UTC−7).

Denver · MDT (UTC−6)Arizona · MST all year (UTC−7)

Quick answers

Selected conversion

9:00 am09:00 ADT (UTC-3), Oct 18, 2026 = 6:00 am06:00 MDT (UTC-6), Oct 18, 2026

ADT is 3 hours ahead of MDT.

AST and MST examples

These examples always use AST (UTC-4) and MST (UTC-7). They stay the same when you change the date or time above. The selected conversion may differ when daylight saving time applies.

AST is 3 hours ahead of MST.

Most of Arizona stays on MST (UTC-7) all year. Places such as Denver use MDT (UTC-6) during daylight saving time, one hour later. For daylight-time examples, use ADT to MDT.

How the time difference changes

For most of the year, ADT is 3 hours ahead of MDT. From Nov 1, 2026 to Mar 14, 2027, AST is 3 hours ahead of MST. Briefly around clock changes, ADT is 4 hours ahead of MST or AST is 2 hours ahead of MDT.

For most of the year, working hours of 9:00 am to 6:00 pm in ADT correspond to 6:00 am to 3:00 pm in MDT. At other times of the year, working hours of 9:00 am to 6:00 pm in AST correspond to 6:00 am to 3:00 pm in MST.For most of the year, working hours of 09:00 to 18:00 in ADT correspond to 06:00 to 15:00 in MDT. At other times of the year, working hours of 09:00 to 18:00 in AST correspond to 06:00 to 15:00 in MST.

Compare AST time with MST time, including current local time, UTC offsets, time difference, daylight saving changes, working hours, and shareable meeting times.

Time Difference

Atlantic Daylight Time is 3 hours ahead of Mountain Daylight Time.
9:00 am09:00 in ADT is 6:00 am06:00 in MDT

ADT to MDT call time
Best time for a conference call or a meeting is between 12pm-6pm12:00-18:00 in ADT which corresponds to 9am-3pm09:00-15:00 in MDT

9:00 am09:00 Atlantic Daylight Time (ADT). Offset UTC -3:00 hours
6:00 am06:00 Mountain Daylight Time (MDT). Offset UTC -6:00 hours

9:00 am09:00 ADT / 6:00 am06:00 MDT

Choose an hour to update the converter. Day labels such as “next day” or “+1” are relative to the selected date in the first column. Use Up and Down arrow keys, Home or End to move between hours, then Enter or Space to select.

Hourly time comparison
ADT	MDT
	9:00pm previous day
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	21:00 previous day
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Atlantic Standard Time

Offset: AST is 4 hours behind Greenwich Mean Time (GMT).

Countries using this time zone: Anguilla, Antigua and Barbuda, Aruba, Barbados, Bermuda, British Virgin Islands, Canada, Caribbean Netherlands, Curaçao, Dominica, Dominican Republic, Greenland, Grenada, Guadeloupe, Martinique, Montserrat, Puerto Rico, Saint Barthélemy, Saint Kitts and Nevis, Saint Lucia, Saint Martin, Saint Vincent and the Grenadines, Sint Maarten, Trinidad and Tobago, US Virgin Islands

Principal Cities: The largest city in the AST time zone is Santo Domingo in the Dominican Republic, with about 2.202 million people. Other major cities in this time zone include Santiago de los Caballeros, Halifax, San Juan, San Pedro de Macorís.

French: HNA - Heure Normale de l'Atlantique

Spanish: AST - Tiempo Estándar del Atlántico

Map of the areas on Atlantic Standard Time (AST) — AST all year AST part of the year
Today the lighter areas are on UTC−3 (ADT).
Open the world time zone map © OpenStreetMap

Daylight Saving: This is a standard time zone. During summer, some places set clocks forward one hour for daylight saving time and observe Atlantic Daylight Time (ADT).

End: Atlantic Standard Time (AST) ended on Sunday, March 8, 2026 at 2:00 am02:00 local time and clocks were set one hour forward to Sunday, March 8, 2026, 3:00 am03:00 local daylight time instead. Standard time ends annually on the second Sunday of March.

Start: Atlantic Standard Time (AST) starts on Sunday, November 1, 2026 at 2:00 am02:00 local time and clocks are set one hour back to Sunday, November 1, 2026, 1:00 am01:00. Standard time starts annually on the first Sunday of November.

See Canada daylight saving time and upcoming clock changes worldwide.

AST representations, usage and related time zones

W3C/ISO-8601: International standard covering representation and exchange of dates and time-related data

-04 - basic short
-0400 - basic
-04:00 - extended

Email/RFC-2822: Internet Message Format Date Standard, typically used for timestamps in email headers

-0400 - sign character (-) followed by a four digit time providing hours (04) and minutes (00) of the offset. Indicates an offset of four hours to the west of the zero meridian.

Military/NATO: Used by the U.S. military, Chinese military and others

Quebec - Military abbreviation for AST
Q - short form of 'Quebec'

IANA/Olson: Reflects AST time zone boundaries defined by political bodies, primarily intended for use with computer programs and operating systems

America/Anguilla
America/Antigua
America/Aruba
America/Barbados
America/Blanc-Sablon
America/Curacao
America/Dominica
America/Glace_Bay
America/Goose_Bay
America/Grenada
America/Guadeloupe
America/Halifax
America/Kralendijk
America/Lower_Princes
America/Marigot
America/Martinique
America/Moncton
America/Montserrat
America/Port_of_Spain
America/Puerto_Rico
America/Santo_Domingo
America/St_Barthelemy
America/St_Kitts
America/St_Lucia
America/St_Thomas
America/St_Vincent
America/Thule
America/Tortola
America/Virgin
Atlantic/Bermuda
Canada/Atlantic

AST Abbreviation: Several time zones share AST abbreviation and it could refer to these time zones

Atlantic Standard Time - UTC -4
Arabia Standard Time - UTC +3

Time zones with the GMT -4 offset:

AST - Atlantic Standard Time
EDT - Eastern Daylight Time
AMT - Amazon Time
BOT - Bolivia Time
CDT - Cuba Daylight Time
CLT - Chile Standard Time
GYT - Guyana Time
HLV - Hora Legal de Venezuela
Q - Quebec Time Zone
VET - Venezuelan Standard Time

Mountain Standard Time

Offset: MST is 7 hours behind Greenwich Mean Time (GMT).

Countries using this time zone: Canada, Mexico, United States

Principal Cities: The largest city in the MST time zone is Phoenix in the United States, with about 1.65 million people. Other major cities in this time zone include Ciudad Juárez, Hermosillo, Culiacán, Denver.

French: HNR - Heure Normale des Rocheuses

Spanish: MT - Zona Pacífico

Map of the areas on Mountain Standard Time (MST) — MST all year MST part of the year
Today the lighter areas are on UTC−6 (MDT).
Open the world time zone map © OpenStreetMap

Daylight Saving: This is a standard time zone. During summer, some places set clocks forward one hour for daylight saving time and observe Mountain Daylight Time (MDT).

End: Mountain Standard Time (MST) ended on Sunday, March 8, 2026 at 2:00 am02:00 local time and clocks were set one hour forward to Sunday, March 8, 2026, 3:00 am03:00 local daylight time instead. Standard time ends annually on the second Sunday of March.

Start: Mountain Standard Time (MST) starts on Sunday, November 1, 2026 at 2:00 am02:00 local time and clocks are set one hour back to Sunday, November 1, 2026, 1:00 am01:00. Standard time starts annually on the first Sunday of November.

See USA daylight saving time and upcoming clock changes worldwide.

MST representations, usage and related time zones