Apple Health & timezones: what we know and current limitations

[bartmichalak]

Context

We need timezone information to correctly interpret when health events (e.g. sleep) happen in the user's local time. This discussion documents what we've learned about how Apple Health handles timezones.

Key Facts

1. Apple Health stores everything in UTC

There is no timezone offset attached to standard health records.

2. HKMetadataKeyTimeZone exists but is mostly null

Apple Health has a metadata field HKMetadataKeyTimeZone - but in practice:

Record type	Timezone available?
Regular records (steps, heart rate, etc.)	Never
Sleep records (Apple Watch)	~100% (only manually added ones are missing it)
Workout records	Partial - only those recorded by Apple Watch have it; manual/third-party app entries don't

Based on a full sync test: out of 2026 sleep records, only 2 were missing timezone (both manually added). For workouts, 5 out of 19 had timezone - all from Apple Watch.

Ref: Stack Overflow - HKMetadataKeyTimeZone is always nil

3. Current device timezone is not a reliable fallback

We decided not to fall back to the current device timezone because it would falsify historical data - the user may have been in a different timezone when the data was recorded.

4. Open questions

• How big of a problem is this in practice for our users?
• Can we use Sleep record timezones to infer timezone for other records from the same time period?
• Are there other creative approaches to solve this?