Linux and the BSDs have two basic ways of keeping time.
In a "posix" mode the system clock counts UTC seconds since the POSIX Epoch.
In a "right" mode the system clock counts TAI seconds since the POSIX Epoch.
Both Linux and the BSDs in theory can operate in either mode.
In practice, the "right" capability, albeit available in the source, is not enabled in FreeBSD and TrueOS; and Linux operating systems usually operate in "posix" mode, because programs like
ntpd demand it.
(OpenBSD has a "right" capability, though; including working
posix2time() library functions.)
tai64nlocal commands all deal in TAI.
They have to convert TAI timestamps into system time and they have to convert the system clock into TAI timestamps.
They do so by attempting to deduce, from the value of the
TZDIR environment variables, whether the system is in "posix" or in "right" mode.
From this deduction they know how to convert between the the system clock time and TAI.
In "right" mode this is a simple addition or subtraction, as the POSIX Epoch is 1970-01-01 00:00:10 TAI.
In "posix" mode they have to account for the insertion of leap seconds, which count as TAI ticks but do not count as UTC ticks, and which are not inserted according to any algorithmic pattern.
The programs in daemontools, daemontools-encore, and runit do not correctly calculate TAI. They assume that the system clock is always counting TAI seconds since the Epoch, even if the system is not using a "right" timezone but is running in "posix" mode. The result is that the TAI64N timestamps generated by these toolsets are, as of 2017, 27 seconds behind the actual TAI seconds count since the POSIX Epoch. (The requirement that one must always use a "right" timezone was stated in the original Bernstein documentation, although many people have overlooked it or not realized the significance of what the documentation said.)
The result of that is that mixing those tools with the nosh toolset will produce surprising results, if one is unaware of this difference.
If passed through such tools, the outputs of
tai64n will appear advanced by up to 27 seconds.
Similarly, the output of such tools, passed through
tai64nlocal, will appear retarded by up to 27 seconds.