All-day items are date-only in iCalendar and are stored as UTC midnight of
their calendar date — the convention the tasks and calendar providers read
back (OpenTasks resolves them through DateTime.toAllDay(), which discards the
time and resolves the remaining date against UTC).
formatDate/formatDateTime/formatDateTimeCompact resolved every instant against
ZoneId.systemDefault(), so an all-day value rendered as the *previous* day for
anyone west of UTC: a task stored as 2026-07-20T00:00Z showed as 19 Jul in New
York. East of UTC it happened to land on the right day, which is why this went
unnoticed.
formatDate now takes an allDay flag (defaulting to false, so existing call
sites are unaffected) and all-day values resolve against UTC.
Callers must pass allDay at the call site — agendula and calendula both have
sites that need updating.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jean-Luc Makiola
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