Fix showSixWeeks page() using aligned span instead of /6 heuristic

[Norcy]

Problem

When showSixWeeks is true, page() used (daysBefore + monthLength) / 6 >= 6 to decide whether to extend to by 7 days. This does not match the real week-aligned cell count and caused:

• An extra 7th row in some months (e.g. March 2026 with firstDay=1).
• Missing trailing next-month cells when the heuristic skipped the extension (e.g. May 2026 stayed at 5 weeks).

Solution

After aligning from and to to week boundaries, extend by one week only when from.diffDays(to) + 1 < 42. Remove the /6 heuristic and the unused daysBefore variable.

Testing

yarn test src/dateutils.spec.js

Made with Cursor

[CreatlV]

Thanks for tackling this — using the aligned page span is much clearer than the current / 6 heuristic.

There is one remaining edge case: February 2026 with firstDay={0}. After alignment, from through to spans 28 days, so adding a single week produces 35 days (five rows), not 42.

Could the extension add the exact missing number of days instead?

const daysOnPage = from.diffDays(to) + 1;

if (showSixWeeks && daysOnPage < 42) {
  to.addDays(42 - daysOnPage);
}

That gives 28 → 42, 35 → 42, and leaves 42 unchanged. It would be good to add regression cases for February 2026 with a Sunday start, May 2026 with a Monday start, and a month that naturally spans six rows. I also checked this calculation across 1900–2100 with firstDay values 0–6; every showSixWeeks page resolves to 42 days.