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Leap seconds
UTC has inserted 27 leap seconds since 1972, so a day is occasionally 86,401 seconds; Google and AWS 'smear' the leap second over 24 hours, disagreeing with UTC by up to ~0.5 s. Negligible for hour-of-week bucketing, but real for high-precision joins. (Leap seconds are being phased out by ~2035.)
Class
Case to account for
Affects
2 conversions
01
Detection
Sub-second precision required near a leap-second insertion.
02
Mitigation
01
State the clock model (UTC vs TAI vs smeared). For hour-of-week the error is sub-second — declare it negligible rather than silent.
03
Affected conversions
Timestamp + zonehour-of-week slot
OPENSlot + ISO weekUTC interval
OPENMore in Instants
Epoch and unit confusion