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Id: 126122
Status: new
Priority: 0/
Queue: perl6

Owner: Nobody
Requestors: zefram [at]

Severity: (no value)
Tag: Bug
Platform: (no value)
Patch Status: (no value)
VM: (no value)

Subject: [BUG] Instant.from-posix has faulty pre-1961 leap second knowledge
Date: Sun, 20 Sep 2015 19:42:39 +0100
From: Zefram <zefram [...]>
To: rakudobug [...]
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Reusing again the function I explained in [perl #126119], let's look at 1950: $ ./perl6 -e 'sub leaps-between ($a, $b) { (Instant.from-posix($b) - Instant.from-posix($a)) - ($b - $a) }; say leaps-between(-631152000, -599616000)' 0 Allegedly no leaps. As with [perl #126121], this is not correct. Unlike the situation in [perl #126121], prior to 1961 there is no definition of UTC. This breaks the POSIX definition of time_t. There are two reasonable approaches to this situation. The first is to say that POSIX time values are meaningless prior to 1961. If that route is taken, Instant.from-posix() must signal an error for such time_t values. The other approach is to tacitly modify the POSIX definition for the pre-UTC era. time_t values are clearly based on some version of UT, and in the pre-UTC era this can be taken to be UT1. If the resolution to [perl #126121] is to use something other than UTC for the [1961, 1972) era, then for consistency that choice would have to be extended to the pre-1961 era also. (If rubber-seconds UTC is used for [1961, 1972), the choice remains open for the pre-1961 era.) If a non-UTC UT is used prior to 1961, another problem soon arises. Instant values are supposedly counting atomic seconds, so are based on TAI. But TAI is only defined as far back as atomic clocks have been in continuous operation: back to mid-1955. Prior to that there is no TAI. We could therefore say that Instant values can't be constructed for that era. This would affect all ways of constructing Instant values: not just .from-posix(), but also subtraction from an acceptable Instant. But TAI is, in this context, easier to substitute for than UTC. A time scale sufficiently TAI-like for our purposes can be extended back based on TT, the theoretical ideal time scale that TAI attempts to realise. The correspondence between UT1 and TT can be reasonably estimated for times many centuries before the present. Such estimates, ultimately using a quadratic formula such as I proposed in [perl #126119], could form the basis of an Instant.from-posix() implementation for the pre-TAI era. Estimates of this nature would almost always yield non-zero `leap counts' (really, slippage between UT and (pseudo-)atomic time). The slippage was certainly non-zero in 1950. Leap counts would also normally be fractional, unless some synthetic proleptic integral-leaps UTC were used rather than UT1. -zefram

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