For a given city, the (physical location in a given) timezone is not absolute; rather it’s a choice.

NYC (along with 23 U.S states) is physically in Eastern time zone, whereas EST and EDT are the “observed timezones”. Some locations in the same physical timezone choose to observe EST only, not EDT.

EDT and EST do not define physical regions. The physical regions that observe  EDT also observe EST, but not conversely, because some physical regions observing EST never observe EDT.

As another example, Pacific timezone cities (like LA) observe both PST and PDT.

Analogy: students.. A college student is usually in exactly one school, but some students are in Engineering school and also in Law school.

— 12H lag, most of the year.

• EDT is 12H behind SGT .. for about 8 months March to Nov
• EST is 13H behind SGT .. for about 4 months

Note daylight Saving starts in Spring…. spring-forward from 2am to 3am in Mar, when the lag Shortens/Shrinks to 12H

Spring forward and stay 8 months, then fall back and stay 4 months (approx.)