Fix NaiveDateTime.diff/3 over-counting incomplete units#15583
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josevalim merged 1 commit intoJul 10, 2026
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`NaiveDateTime.diff/3` truncated each operand to the requested unit and only then subtracted, i.e. it computed `floor(t1) - floor(t2)` instead of `floor(t1 - t2)`. When the subtrahend's sub-second fraction exceeded the minuend's, this rounded the result *up*, contradicting the documented guarantee that "Fractional results are [...] truncated" and the example that it "rounds incomplete days to zero". For instance an elapsed span of 86399.5s (less than one day) returned `1` for `:day`, and a 0.5s span returned `1` for `:second`. The sibling `DateTime.diff/3`, which carries the identical doc line, returned `0` for the same wall-clock instants because it computes the difference at the `:microsecond` base first and truncates once via `System.convert_time_unit/3`. Align `NaiveDateTime.diff/3` with `DateTime.diff/3` so both agree on identical instants. All existing doctests are preserved. Assisted-by: Claude Code:claude-opus-4-8
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NaiveDateTime.diff/3truncated each operand to the requested unit and only then subtracted, i.e. it computedfloor(t1) - floor(t2)instead offloor(t1 - t2). When the subtrahend's sub-second fraction exceeded the minuend's, this rounded the result up, contradicting the documented guarantee that "Fractional results are [...] truncated" and the example that it "rounds incomplete days to zero".For instance an elapsed span of 86399.5s (less than one day) returned
1for:day, and a 0.5s span returned1for:second. The siblingDateTime.diff/3, which carries the identical doc line, returned0for the same wall-clock instants because it computes the difference at the:microsecondbase first and truncates once viaSystem.convert_time_unit/3.Align
NaiveDateTime.diff/3withDateTime.diff/3so both agree on identical instants. All existing doctests are preserved.Assisted-by: Claude Code:claude-opus-4-8