While "3...2...1...leap" was mentioned on the mailing list in 1998, support for rolling leap seconds goes all the way back to the 1.1, 1988-01-25 version of the leapseconds file (included below). Folks looking for documentation of "3...2...1...leap" in New York might focus on 1987 or, before that, 1972 through 1979. (My guess 1987.)

    --ado

# @(#)leapseconds 1.1

# Allowance for leapseconds added to each timezone file.

# The correction (+ or -) is made at the given time, so lines
# will typically look like:
# Leap YEAR MON DAY 23:59:60 + R/S
# or
# Leap YEAR MON DAY 23:59:59 - R/S

# If the leapsecond is Rolling (R) the given time is local time
# If the leapsecond is Stationary (S) the given time is GMT

# Leap YEAR MONTH DAY HH:MM:SS CORR R/S
Leap 1972 Jun 30 23:59:60 + S
Leap 1972 Dec 31 23:59:60 + S
Leap 1973 Dec 31 23:59:60 + S
Leap 1974 Dec 31 23:59:60 + S
Leap 1975 Dec 31 23:59:60 + S
Leap 1976 Dec 31 23:59:60 + S
Leap 1977 Dec 31 23:59:60 + S
Leap 1978 Dec 31 23:59:60 + S
Leap 1979 Dec 31 23:59:60 + S
Leap 1981 Jun 30 23:59:60 + S
Leap 1982 Jun 30 23:59:60 + S
Leap 1983 Jun 30 23:59:60 + S
Leap 1985 Jun 30 23:59:60 + S
Leap 1987 Dec 31 23:59:60 + S


On Mon, Sep 13, 2021 at 3:57 AM Guy Harris <gharris@sonic.net> wrote:
On Sep 13, 2021, at 12:18 AM, Paul Eggert via tz <tz@iana.org> wrote:

> Thanks for the background.

You participated in the tz mailing list thread:

        http://mm.icann.org/pipermail/tz/1998-May/010164.html

in which he mentioned that back in 1998:

        http://mm.icann.org/pipermail/tz/1998-May/010159.html

(things I learned from a Web search today; no, I didn't remember that discussion, either).

(And, on one of the topics you addressed in your message:

>    What Epoch do systems that use leap seconds use?
>
> In practice, such systems define 1972-01-01 00:00:00Z == (time_t) 63072000,
> i.e. it's as if there were no leap seconds before 1972.

Presumably systems that *don't* use leap seconds use the same definition.)