Oura and Polar recovery rows are bucketed on the local day of their raw
measuredAt, on the assumption that both stamp the wake morning. Polar
always anchors at UTC midnight of the wake date and Oura falls back to the
same anchor when no real wake instant is present, so for a user in a far
negative UTC zone that anchor reads as the previous local evening and a
single night can split across two recovery days near the date line. A
correct fix needs an anchor-kind signal threaded through the recovery
read so the date-anchored sources read their wake day in UTC while a real
wake instant stays local — out of scope here. Document the assumption at
the bucketing site; positive-UTC users are unaffected.
Marc-André Bombeck
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