Restructure date calculations to avoid manual modular logic

[Mitch-A6]

Are you removing, renaming or moving a folder in this PR?

• No, I'm not changing any folders (skip to TeamSites and delete the rest of this section)

TeamSites

Did you change site-wide styles, platform utilities or other infrastructure?

• No
• Yes, and I used the proxy-rewrite steps to test the injected header scenario

Summary

• Replace manual string-based date math with JS date manipulation to stabilize tests and logic.
• The previous implementation calculated December dates such that e.g., 2024-12-27 became 2025-00-27, which was incorrectly considered above 2024-12-31. This PR replaces the manual date + year manipulation and modular math with JS's Date.setMonth() logic. As a pleasant surprise, date.setMonth(date.getMonth()+2) even correctly shifts across year boundaries.
• Health Care Application 10-10EZ/CG Team

Related issue(s)

• Slack discussion in #octo-ves-healthcare-enrollment at https://dsva.slack.com/archives/C06FDF2EN0H/p1735333763795659

Testing done

• Before this fix, this would incorrectly determine dates after December 25th to be unpaused rather than paused.
• Specs and automated tests passing, due to this fix
• ⚠️ direct testing?

What areas of the site does it impact?

⚠️ (Describe what parts of the site are impacted if code touched other areas)

Acceptance criteria

Quality Assurance & Testing

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Linting warnings have been addressed
• Documentation has been updated (link to documentation *if necessary)
• Screenshot of the developed feature is added
• Accessibility testing has been performed

Error Handling

• Browser console contains no warnings or errors.
• Events are being sent to the appropriate logging solution
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)

Authentication

• Did you login to a local build and verify all authenticated routes work as expected with a test user

Requested Feedback

(OPTIONAL) What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?

[IGallupSoCo]

LGTM