Simplify cookie expiry time calculation

[pbearne]

Pull Request Type

• I have checked there is no other PR open for the same change.

This is a:

• Bug fix
• New feature
• Documentation improvement
• [ x] Code quality improvement

Context

Detailed Description

Replaces strtotime with DateTimeImmutable for calculating the cookie expiry time. This ensures more consistent behavior and improves code readability.

Quality assurance

• [ x] This change does NOT contain a breaking change (fix or feature that would cause existing functionality to change).
• [ x] I have commented my code, particularly in hard-to-understand areas.
• I have added unit tests to accompany this PR.
• [ x] The (new/existing) tests cover this PR 100%.
• I have (manually) tested this code to the best of my abilities.
• [x ] My code follows the style guidelines of this project.

Documentation

For new features:

• I have added a code example showing how to use this feature to the examples directory.
• I have added documentation about this feature to the docs directory.
  If the documentation is in a new markdown file, I have added a link to this new file to the Docs folder README.md file.