The offset is not fixed, which is the whole problem
UTC never moves. Eastern does. From the second Sunday in March to the first Sunday in November the eastern United States runs at UTC−4; the rest of the year it runs at UTC−5. So converting a UTC timestamp needs the date as well as the time, and any code that hardcodes minus five will be an hour wrong for about eight months of the year.
Reading server logs and ISO timestamps
Almost all logging, database and API timestamps are stored in UTC, usually in ISO 8601 form ending in a Z, such as 2026-03-15T14:30:00Z. That trailing Z means UTC exactly. Converting to Eastern for an incident report or a customer email is where the mistakes creep in, especially for events near midnight UTC, which fall on the previous evening in Eastern.
Midnight UTC is the previous day in America
00:00 UTC is 8:00 PM the previous evening in Eastern during summer, or 7:00 PM in winter. Daily aggregation jobs that run at midnight UTC therefore cut the American day in the middle of the evening, which is a common source of confusion in reporting dashboards.
GMT is not quite a synonym
People often write GMT when they mean UTC. For practical purposes the two are within a second of each other, but GMT is also the winter time of the United Kingdom, which switches to BST in summer. UTC never does. If a system says UTC, take it literally.