IANA tz database · America/New_York · zdump −v
Every shift in New York's offset from universal time since 1776 — from the city's own local mean time, through the “Day of Two Noons,” two world wars, a twenty-year patchwork, and the modern spring-forward rhythm. Read straight off the operating system's timezone rules.
Railroads impose standard time zones. At noon, New York's clocks jump back ~4 minutes from local mean time (−4:56:02) to a round −5:00 EST.
The Standard Time Act brings the first federal spring-forward — the sawtooth begins, oscillating between −5 (winter) and −4 (summer).
For 3½ straight years the clock holds at −4:00 — Eastern War Time, then Eastern Peace Time — never falling back. The one long plateau on the summer level.
No federal rule: start and end dates drift year to year by local option, the reason the teeth in this stretch are uneven in width.
Congress standardizes DST nationwide — last Sunday in April to last Sunday in October — and the sawtooth snaps into a regular, even beat.
Amid the oil embargo, Nixon-era law puts the country on daylight time through winter — two brief off-season spikes to −4 in January 1974 and February 1975.
DST widens to the second Sunday in March through the first Sunday in November. The summer teeth grow noticeably wider from here on.