You might find Steve Allan's analysis of how many leap-seconds there
could be interesting:
https://www.ucolick.org/~sla/leapsecs/dutc.html
-Brooks
On 2026-07-17 06:34 PM, Paul Eggert via tz wrote:
On 2026-07-17 14:02, Doug Ewell via tz wrote:
I sure hope I’ve never built software that subtracts one second by
skipping two.
Good catch. This reminds me of the error in C89, which allowed two
adjacent positive leap seconds. That mistake wasn't fixed until C99
and POSIX.1-2001, and a lot of popular software still supports the
mistake. For example, although Python 3.14.6 rejects out-of-range
seconds counts, it allows the counts up through 61, i.e., it mimics
the C89 error:
>>> import time
>>> time.asctime((2026, 12, 31, 23, 59, 59, 3, 365, 0))
'Thu Dec 31 23:59:59 2026'
>>> time.asctime((2026, 12, 31, 23, 59, 60, 3, 365, 0))
'Thu Dec 31 23:59:60 2026'
>>> time.asctime((2026, 12, 31, 23, 59, 61, 3, 365, 0))
'Thu Dec 31 23:59:61 2026'
>>> time.asctime((2026, 12, 31, 23, 59, 62, 3, 365, 0))
Traceback (most recent call last):
File "<python-input-4>", line 1, in <module>
time.asctime((2026, 12, 31, 23, 59, 62, 3, 365, 0))
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: seconds out of range
For what it's worth, in a hypothetical and impractical approach where
we continued to insert one or two leap seconds as needed, possibly in
every minute, I estimate we could keep that up for over 100 million
years before an Earth day would be so long that we'd at least
occasionally have to go to three inserted leap seconds per minute.
It's unlikely that this sort of thing is what C89's authors and
Python's developers were thinking of.
