tzdata 2026a Is More Than a Moldova Fix
IANA released tzcode and tzdata 2026a on March 2, 2026. The data correction was concrete: Moldova has followed European Union transition times since 2022, so the release fixed its upcoming March 29 clock change and other affected timestamps. The same release also changed tzcode build behavior around leap-second-aware files, which makes it relevant beyond one timezone rule update.
POSIX Time and Leap-Second Time Are Different Paths
For Unix timestamps, the important split is between ordinary POSIX-style time and a leap-second-aware clock. RFC 9636 defines UNIX time as integer seconds since the POSIX epoch without counting leap seconds. IANA's timezone documentation describes the default "posix" configuration the same way, while the optional "right" configuration lets an internal time_t clock count every TAI second. Those paths agree before the first leap second in 1972 and diverge afterward.
The right TZif Set Is No Longer Installed by Default
tzcode 2026a changes the Makefile default so it no longer installs the alternate TZif files intended for system clocks that count leap seconds. IANA still documents how packagers can request the previous mixed install with REDO=posix_right or a leap-second-oriented install with REDO=right_only. The practical effect is that software depending on a default tzcode build should not assume those alternate "right" files are present after the 2026a update.
tzcode Can Disable Runtime Leap-Second Support
The release also adds a TZ_RUNTIME_LEAPS=0 compile-time path for tzcode. IANA describes that choice as closer to POSIX behavior and smaller at runtime, while also warning that it does not support the leap-second behavior covered by RFC 9636. That tradeoff matters for specialized TZif readers and packagers even though ordinary Unix timestamp arithmetic normally follows the leap-second-ignoring POSIX model.
What Timestamp and Timezone Users Should Check
Applications that store Unix epoch seconds and convert local time with IANA region IDs stay on the normal POSIX path, but they still need current tzdata when local rules change, including Moldova's corrected transition data. Systems that intentionally consume leap-second records, ship "right" zone files, or validate TZif handling should audit their tzcode build flags and package contents after 2026a. IERS Bulletin C 71 says no leap second will be introduced at the end of June 2026, but build defaults can change before the next real leap-second event arrives.