# New Ticket Created by Zefram
# Please include the string: [perl #126119]
# in the subject line of all future correspondence about this issue.
# <URL: https://rt.perl.org/Ticket/Display.html?id=126119 >
The Instant class supposedly represents times on the TAI time scale,
with subtraction of Instants yielding counts of atomic seconds.
The corresponding difference of POSIX time_t values yields a count
of non-leap seconds. The difference between these two differences,
for corresponding endpoints, therefore yields a count of leap seconds.
Instant.from-posix() will conveniently perform the translation. So here's
some code to count the leap seconds that occur in an interval specified
by time_t values:
$ ./perl6 -e 'sub leaps-between ($a, $b) { (Instant.from-posix($b) -
Instant.from-posix($a)) - ($b - $a) }; say leaps-between(time - 1000000000,
time); say leaps-between(time, time + 1000000000)'
14
0
The first of the time intervals on which I've tried that runs from January
1984 to today, and the count of 14 leap seconds is historically correct.
The second interval runs from today to May 2047... and will almost
certainly contain a non-zero number of leap seconds. The count of zero
is bogus.
In reality, we don't know how many leap seconds there will be in
the next gigasecond. We can guess: anything from five to twenty
is defensible. But there is no value that we can say is certainly
correct. So Instant.from-posix(time + 1000000000) cannot produce any
definitely-correct value. It really ought to signal an error.
If it is really intended to guess, in cases where a definitive answer is
available, then the guess that it is making is crap. To make a reasonable
guess, use a quadratic formula based on the observed tidal braking,
and quantise it to one-second leaps on appropriate month boundaries.
A finer guess could be made for the next few decades by extrapolating
from recent decades' measurements.
Also, if it's a guessing function, it ought to be named in a way that
clues in the user. "from-posix-best-guess" or so. It would also be
sane to have both kinds of conversion function.
-zefram
