The admin group-stage schedule form submitted the raw datetime-local
string, which the server parsed with new Date() in the server's timezone
(UTC), so an admin's local wall-clock time was stored as if it were UTC.
Downstream displays then correctly converted UTC->local, shifting times
by the admin's UTC offset (e.g. 7 PM PDT showed as 12 PM).
Mirror the existing playoff "add-game" form: convert local->UTC on the
client via localDateTimeToUtcIso into a hidden field on submit, and
render the stored UTC value back into the input in the browser's local
timezone via a client-only effect (avoids SSR hydration mismatch).
Adds utcIsoToLocalDateTime helper (inverse of localDateTimeToUtcIso)
with unit tests covering nullish/invalid input and a tz-independent
round-trip.
Note: matches saved before this fix must be re-saved once in admin to
correct their stored values.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0125nZohVD2Cpoq3Q9f4jeds
516b4e7131