Mst in est

How it works

MST is UTC−7. EST is UTC−5. MST in EST is a two-hour step: add two hours going from Mountain Standard to Eastern Standard. Subtract two hours going the other way. 9 a.m. MST is 11 a.m. EST. 4 p.m. EST is 2 p.m. MST. Write the zone on the invite. A bare “noon” is how Denver and New York both show up at the wrong door.

MST EST as a pair of letters is only safe in winter for most of the U.S. Mountain and Eastern states, and year-round for most of Arizona. In summer, places that use daylight time switch labels: MDT is UTC−6, EDT is UTC−4. The gap stays two hours if both sides change. MST and EST as frozen winter letters will lie in July if you apply them to Denver and New York, because those cities are then on MDT and EDT.

MST and EST in the same sentence should mean those two offsets, not “Mountain Time versus Eastern Time” as seasons. If you do not want to track S versus D, say Mountain Time and Eastern Time, or name cities. Software should use America/Denver, America/Phoenix, and America/New_York rather than a raw MST string.

Arizona is the special case

Most of Arizona stays on Mountain Standard Time all year and does not use daylight time. The Navajo Nation, in the northeast of the state, does use daylight time. That split is why “Arizona time” is not one offset in summer. Phoenix in July is still MST (UTC−7). New York in July is EDT (UTC−4). The gap is then three hours, not two. Noon in Phoenix is 3 p.m. in New York in that season. In January both Phoenix and New York are on standard time, and the gap is two hours again.

MST (UTC−7) to EST (UTC−5): add two hours. EST to MST: subtract two hours. MDT to EDT: still two hours, both on daylight. Arizona MST to EDT in summer: add three hours. Idaho, Oregon, and a few other states also split zones. Convert from a city or from an IANA name, not from a state line. MST to EST is arithmetic on offsets. A state name is not an offset. Two hours is not a rounding error. Denver lunch is already afternoon in New York.

How to convert without mixing the letters

If the labels look clean, add or subtract two. If one side might be Arizona in summer, or a place that keeps Eastern Standard year-round, convert through UTC. MST is −7, EST is −5, MDT is −6, EDT is −4. Two labels with the same offset are the same instant with different names. MDT and EST are UTC−6 and UTC−5: still one hour apart, which is a different problem from MST in EST.

Travel days are when people keep the home zone in their head. You land in Atlanta and still add two hours for “Mountain” as if you had not moved. Reset the phone to local, then convert the other city from there. The mst to est step does not change. Your location does.

Meetings that need both rooms awake belong in late morning Mountain and early afternoon Eastern, or early afternoon Mountain and late afternoon Eastern. A 7 a.m. EST standup is 5 a.m. MST, which is an alarm, not a meeting. A 6 p.m. MST “quick chat” is 8 p.m. EST, which is evening on the East Coast. Place the call in the overlap you actually have.