The density the owner chose. Newest day first, weekday named because that is
what a person recognises, and order within a day preserved so a row just
entered stays where it was put.
Two cases the obvious implementation gets wrong, both tested:
A finance plan's later slices belong in this month — money moves this month —
but the purchase's own date does not. Filing them by date would have put
"14 August" inside the September list, so they group separately, last, under
"Financed from earlier months".
A purchase date may legally be "YYYY-MM" with no day at all, which gets its
own trailing group rather than a heading reading "NaN".
The weekday is computed in UTC on both ends. `new Date("2026-09-01")` parses
as UTC midnight, so `getDay()` anywhere west of Greenwich reports the previous
day and every heading would be wrong by one for the user. A test pins three
timezones.
Trade-off worth stating: headings cost vertical space on a day with a single
purchase, and only pay for themselves when a day has several. PRODUCT.md says
the owner records in batches — "a grocery trip is twenty lines entered in one
sitting" — so real data should sit on the paying side of that.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
