DateTime ↔️ timezone package: Interoperability of UTC, Local, and IANA Zones

Goal

Convert a core DateTime instance into a timezone‑aware representation using the timezone package, while preserving accurate offsets, including during DST transitions.

Constraints & Uncertainty

• The core DateTime class exposes only UTC or the system’s local zone; it has no IANA zone identifier.
• Converting to a named zone requires explicit loading of tzdata via loadTimeZones(), otherwise a runtime exception occurs.
• Different platform implementations (Android, iOS, macOS, Linux) may resolve the system’s local zone differently, leading to inconsistent offsets when calling toLocal() on a timezone‑converted instance.
• Offsets around DST transitions can vary if the tzdata version differs between environments, making deterministic behavior hard to guarantee.

Specific Questions

1. Which pattern best preserves the original zone information when mapping a core DateTime to a TimeZone object?
2. How can we guarantee deterministic offsets across platforms when converting a UTC DateTime to an IANA zone during a DST transition?
3. What best practices mitigate runtime failures caused by missing or outdated tzdata when using the timezone package in production apps?