Fwd: Question about time period transition points

Mike Giroux rmgiroux at gmail.com
Mon Feb 14 19:36:58 UTC 2011

Wise ones of the TZ list,

A (quite academic, but possibly interesting) question came up here as
we're trying to document the behaviour of our time zone code.

We're trying to figure out whether the transition time is reached or skipped.

For example, for these 2 rules,
# Rule  NAME    FROM    TO      TYPE    IN      ON      AT      SAVE    LETTER/S
Rule    TC      1979    1986    -       Apr     lastSun 2:00    1:00    D
Rule    TC      1979    2006    -       Oct     lastSun 2:00    0       S

does the clock reach "2:00" on the transition days?

In that case, the transition would go from
"2:00(S)" -> "3:00(D) + epsilon" when moving forward,
"2:00(D)" -> "1:00(S) + epsilon" when moving back.

Or does it go from
"2:00(S) - epsilon" -> "3:00(D)"
"2:00(D) - epsilon" -> "1:00(S)"

If the transition time IS reached, which "name" would attach to it?

Phrased differently, for the Americas/NewYork transitions this spring:
1) Is the time coordinate value at the instant of transition "2:00 AM" or "3:00
AM" (NIST says "begins at 2:00 a.m. on the second Sunday of March")

2) Is the designation of that coordinate (in the local New York City timezone)
"EDT" or "EST"?



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