Alwaysgaurav1 commented on PR #1780:
URL: https://github.com/apache/commons-lang/pull/1780#issuecomment-5507011770
Thanks for catching this edge case, @garydgregory!
### Root Cause
The issue occurred because `start.add(Calendar.YEAR, 1)` clamped
`2020-02-29` to `2021-02-28`. Since the initial calculation for `days` had
already subtracted `29` (`1 - 29 = -28`), adding the 28 days of February 2021
yielded `-28 + 28 = 0` days instead of `1` day.
### Solution
We don't need to change the normalization direction. Instead, we can unify
the calculation with the rest of the token cascading hierarchy (`y -> M -> d ->
H -> m -> s -> S`):
1. Apply standard hierarchical borrowing for `days` and `months` first
(identical to when `M` is present).
2. If `y` is omitted, roll `years` into `months` (`months += 12 * years;
years = 0;`).
3. If `M` is omitted, roll remaining `months` into `days`:
```java
if (!Token.containsTokenWithValue(tokens, M)) {
while (months > 0) {
days += start.getActualMaximum(Calendar.DAY_OF_MONTH);
months -= 1;
start.add(Calendar.MONTH, 1);
}
}
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