MDT = UTC-6, CDT = UTC-5 (Wait, No, CDT Is Also UTC-6?)

A clock in Denver reads 14:00 in July. A clock in Chicago reads 15:00. Both cities are on daylight time. Denver runs on MDT (UTC-6). Chicago runs on CDT (UTC-5). One hour apart.

The mix-up starts when someone checks a winter offset. Central Standard Time is UTC-6. Mountain Daylight Time is also UTC-6. Same number, different season, different rulebook. People grab the offset and stop there. That is the mistake.

Mountain Daylight Time is the summer clock for the Mountain zone. Central Daylight Time is the summer clock for the Central zone. They never share an offset. MDT and CST share UTC-6, but they operate in opposite halves of the year.

The Offset Confusion Explained

The root of the confusion is simple: time zone abbreviations describe both the zone and whether DST is in effect.

  • Mountain Standard Time (MST): UTC-7 (winter)
  • Mountain Daylight Time (MDT): UTC-6 (summer)
  • Central Standard Time (CST): UTC-6 (winter)
  • Central Daylight Time (CDT): UTC-5 (summer)

When you see "UTC-6", it could mean MDT or CST. The abbreviation tells you which one. Many people check only the offset number and assume "same offset = same zone". That is the trap.

Summer: Denver and Chicago Do Not Match

From the second Sunday in March to the first Sunday in November, Denver is on MDT (UTC-6) and Chicago is on CDT (UTC-5). The clocks are one hour apart.

On 15 July 2026, Chicago is 1 hour ahead of Denver. The solar noon in Denver arrives later. The lived experience of daylight differs, and the wall clock reinforces it: Chicago is always ahead.

A 14:00 meeting in Denver in July is 15:00 in Chicago. Same meeting, different local times. If you book it as "14:00 Mountain Time" and your Chicago colleague books it as "14:00 Central Time," you will miss each other by an hour.

Why They Are Still Different Time Zones

A time zone is more than its current offset. It is a rulebook that includes:

  • The standard offset (winter)
  • The daylight offset (summer)
  • The dates of DST transitions
  • Whether DST is observed at all

MDT belongs to the Mountain Time rulebook: standard UTC-7, daylight UTC-6. CDT belongs to the Central Time rulebook: standard UTC-6, daylight UTC-5.

The two rulebooks never produce the same offset at the same time. MDT (UTC-6) runs in summer. CST (UTC-6) runs in winter. They arrive at UTC-6 from opposite directions and in opposite seasons.

When Does MDT Match CST? Only in Opposite Seasons

In winter, the gap becomes obvious.

On 15 January 2026, Denver is on Mountain Standard Time (UTC-7). Chicago is on Central Standard Time (UTC-6). The clocks are one hour apart. A 09:00 meeting in Denver is 10:00 in Chicago.

MDT and CST both equal UTC-6. But MDT exists only in summer. CST exists only in winter. They never occupy a calendar at the same moment. The offset match is a paper fact, not a scheduling reality.

How to Schedule Between Mountain and Central Zones

Book meetings using the correct IANA zone, not the offset.

In summer (March to November): Denver is on America/Denver (MDT, UTC-6). Chicago is on America/Chicago (CDT, UTC-5). There is a one-hour difference. Set the meeting in the calendar with each attendee's zone. The software handles the conversion.

In winter (November to March): Denver is on America/Denver (MST, UTC-7). Chicago is on America/Chicago (CST, UTC-6). Still one hour apart. Same direction, same gap.

For Arizona: Phoenix stays on America/Phoenix (MST, UTC-7) year-round. In July, a 14:00 Denver meeting is 13:00 in Phoenix. A 15:00 Chicago meeting is also 13:00 in Phoenix. Phoenix matches neither MDT nor CDT in summer. Always specify "Arizona time" or "MST" when inviting Phoenix attendees. Skip the phrase "Mountain Time" for Arizona in summer: it creates ambiguity.

What to skip: Do not use UTC-6 as a shorthand for scheduling. It hides which zone you mean. Do not assume Denver and Chicago show the same clock in any season. They do not.

What to book: When you create a recurring event spanning March or November, set the time in the host's IANA zone. Let the calendar adjust each occurrence. Check the first winter instance and the first summer instance manually. DST transitions catch people who trust the offset.

To see how MST and MDT differ year-round, read MST vs MDT: What's the Difference in Mountain Time?. For the full list of Mountain Time offsets across seasons and exceptions, see MST Time Zone Offsets: UTC-7 and All MST Variations. If you need to convert between Mountain and Central zones, use MST → CST or MST → CDT. And for DST dates and rules, check MST Daylight Saving Time: Dates, Rules, and Exceptions.