DateTime.plus(...) with mixed algebraic signs works incorrectly

[PerfectPixel]

Description

If doing a calculation on DateTime with mixed algebraic signs leads to incorrect results. This is true for example when modifying dates in a way that the target month does not have 30 days or the target year is a leap-year.

Example

// should be 2019-12-09, but is 2019-12-10
DateTime.fromISO('2020-01-08').plus({months: -1, days: 1})

Analysis

Internally, the duration {months: -1, days: 1} is shifted to months, days, milliseconds and during that it is normalized. During normalization, the algebraic signs are changed and the resulting duration is {months: 0, days: -29}. It would be correct if the target month has 30 days, but December has 31 days.

Same issue for leap-years: {years: -4, days: 1} is converted to {years: -3, days: -364}, should be days: -365 if the resulting year is a leap year.

Reproducible example

https://codesandbox.io/s/sleepy-diffie-x761e

Version

Tested with latest Luxon version (1.21.3)

Workaround

Add units one by one in the right order: DateTime.fromISO('2020-01-08').plus({months: -1}).plus({days: 1})

[icambron]

Yeah, I agree that's a bug. We probably just shouldn't normalize there