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fix: properly format fractional seconds for timestamps prior to POSIX epoch (fixes #4975) - #4976
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vitaut merged 1 commit intoOct 18, 2026
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Any thoughts on this patch @vitaut ? Cheers! |
vitaut
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Oct 18, 2026
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Thanks for the fix! |
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Fixes #4975
Summary of Changes
When formatting chrono time points prior to the POSIX epoch (1970-01-01) with subsecond specifiers (e.g.,
:%S),formatter<local_time<Duration>, Char>::formatininclude/fmt/chrono.hpassed negative subsecond durations towrite_fractional_seconds. Becausewrite_fractional_secondscasts duration counts touint64_t, this caused an unsigned integer underflow and produced corrupted outputs like02.18446744073709550739instead of01.123.This patch normalizes negative subsecond durations by adjusting the whole seconds count and shifting fractional units into the positive range (
subsecs += second) before callingwrite_fractional_seconds.Verification
test/chrono-test.cccovering negative millisecond, microsecond, and nanosecond formatting (e.g.epoch - 250ms->59.750,-14398877ms->01.123).chrono-test(34/34 tests) and fullctestsuite (22/22 tests).