fix: return NaN from diff() when an operand is an Invalid Date

[yu2971512385-ui]

Fixes #3186.

The bug

monthDiff in src/utils.js ends with || 0:

return +(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) : (anchor2 - anchor)))) || 0)

When one operand is an Invalid Date, the arithmetic is NaN, and NaN || 0 collapses it to 0. So the month-based units silently return 0 instead of NaN:

dayjs('2018-08-08').diff(dayjs('invalid'), 'month', true) // 0, expected NaN
dayjs('2018-08-08').diff(dayjs('invalid'), 'year',  true) // 0, expected NaN

Every other unit already returns NaN for an Invalid Date operand (they go through plain millisecond arithmetic), and moment.js returns NaN here too — so 'month' | 'quarter' | 'year' are inconsistent.

The fix

Compute the value once and only normalize -0 to 0, letting NaN propagate to diff() unchanged:

const result = -(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) : (anchor2 - anchor))))
return result === 0 ? 0 : result

Behaviour for valid dates is unchanged (-0 is still normalized to 0).

Tests

Added a Difference case asserting diff with an Invalid Date operand is NaN (not 0) for month, quarter and year, in both directions and for the float variants. Verified it fails on the current source and passes with the fix.

🤖 Generated with Claude Code