[mayankdhingra02]

Passing a UTC TimeZone object currently uses the host's local calendar
setters, while the equivalent 'UTC' string uses UTC setters. On a Los
Angeles host, for example:

addMonths(new Date('2025-01-31T00:30:00Z'), 1, {
  zone_name: 'UTC',
  gmt_offset: [0]
});
// Before: 2025-03-01T00:30:00.000Z
// After:  2025-02-28T00:30:00.000Z

Use the normalized zone name when choosing UTC arithmetic in addDays and
addMonths; addYears delegates to addMonths. Passing TimeZone objects
is deprecated in favor of strings, and 'UTC' is the recommended form for
UTC. The timezone objects shipped with the library do not include UTC.

One regression per function lives in its existing UTC Time block, using
process.env.TZ in beforeAll to select a Los Angeles host, following the
repository's test conventions. The cases cover the daylight-saving
transition, month-end clamping and leap-day clamping. All three fail on the
unchanged base and pass with the source fix.

Validation: lint, TypeScript checking, build and the full 2,331-test suite
pass on Node 22.23.4, 24.22.1 and 26.12.1. Coverage also passes on Node 26.
Vitest runs were limited to two workers.

[knowledgecode]

@mayankdhingra02 Thanks for the PR. I was able to reproduce it: because the inner branch checked timeZone instead of zoneName, a zone_name: 'UTC' object fell through to local-time arithmetic. I'd like to take the src/ changes as they are.

For context, none of the TimeZone objects shipped with the library is UTC, and passing timezone objects is deprecated in favor of strings ('UTC' is the recommended form for UTC).

I'd like the tests to follow the repository's conventions. Which would you prefer?

1. You update the tests. Add the cases to the existing UTC Time blocks in tests/addDays.spec.ts and the other affected spec files, and set the host time zone with process.env.TZ in beforeAll as the existing tests do (no child processes needed). One or two cases per function that fail before the fix are enough. I'll merge once the update is in.
2. If you only need the fix merged. I'll merge as is and rewrite the tests myself.

[knowledgecode]

Thanks for updating the tests! LGTM.