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ISO 8601 duration parser and formatter
Read and write ISO 8601 durations like P1DT2H.
ISO 8601-1 durations · weeks/days/time are exact elapsed time · years and months are nominal
P1W · PT30M · -PT90.5S: fractions only on the lowest component, weeks stand alone.
- ISO 8601-1 grammar: weeks are exclusive (P1W, never P1Y2W), fractions only on the lowest-order component, comma or dot as the separator.
- Days and weeks are fixed elapsed time (1 d = 24 h, 1 w = 7 d). Those totals are exact.
- Years and months stay components: no exact millisecond total exists for them, and none is fabricated here.
- Sub-millisecond fractions are truncated, never rounded into the next second.
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