Getting Started

v1.9.0 is a compatibility release: it adds PHP 8.5 support and fixes Carbon 3 compatibility. No new API surface is indicated.
First check composer.json for php and nesbot/carbon constraints. If you are on PHP 8.4 or Carbon 2, this is a low-risk upgrade; if you are moving to PHP 8.5 or Carbon 3, v1.9.0 should be your minimum version.
First use case: upgrade path. Require ^1.9, run composer update <vendor/package> --with-dependencies, then run php artisan about and your test suite.
Look first at the README installation section, the v1.9.0 changelog entry, the package service provider (auto-discovery), and any publishable config file.

Implementation Patterns

Treat v1.9.0 as a drop-in compatibility update. Pin ^1.9 in composer.json and let CI validate the upgrade.
Add PHP 8.5 and Carbon 3 to your CI matrix. If the package supports both Carbon 2 and 3, test both explicitly rather than relying on the lockfile.
Isolate date/time logic behind your own service or adapter. Inject CarbonInterface or use Laravel’s now() / Date facade so Carbon major upgrades do not leak through application code.
In tests, freeze time with $this->travelTo() or Carbon::setTestNow() and assert date math with tolerant comparisons where Carbon 3 changes return types.
If the package exposes config, publish it once and version the file. Use environment variables for runtime knobs so config caching remains predictable.
For queued jobs and scheduled tasks, serialize dates as ISO-8601 and keep timezone handling explicit.

Carbon 3 changed several diffIn* return types and method signatures. Run tests around date differences, timezone conversions, and add/subtract operations; do not assume integer returns.
PHP 8.5 can surface deprecations in transitive dependencies. Run the suite with deprecation reporting enabled and check composer why nesbot/carbon and composer outdated.
If you use Composer platform config, setting "php": "8.5" too early can force resolution to older package versions. Verify all dependencies support PHP 8.5 before changing the platform constraint.
Clear caches after upgrading: php artisan config:clear, php artisan cache:clear, and reset OPcache in production.
If the package does not use Laravel auto-discovery, confirm the service provider and aliases are still registered after the update.
Extension points are likely unchanged: config publishing and container bindings remain the safest integration points. Avoid extending Carbon directly.
Debugging commands: composer show <vendor/package>, composer why-not php 8.5, and vendor/bin/phpunit --display-deprecations.