test: add edge case coverage for dateDiffDays

[adityagautam6102]

Added edge case unit tests for dateDiffDays utility function.

Changes

• Added positive and negative date difference tests
• Added edge case coverage
• Verified same-date behavior

Validation

• Ran Vitest tests successfully

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Hi! Your PR has merge conflicts with the current main branch. Please rebase your branch on latest main and resolve the conflicts:

git fetch upstream
git rebase upstream/main
# resolve any conflicts
git push --force-with-lease origin your-branch

Once resolved, we'll merge it in. Thanks!

[adityagautam6102]

Hi! Please check and let me know if any updates are required...