Vala GLib.DateTime to GTK Calendar integration: managing regional time‑zone conversion

The goal is to reliably convert a GLib.DateTime value that represents an instant in a chosen time zone into the GLib.Date expected by a GTK Calendar widget, ensuring the displayed day, month and year match the instant’s local calendar date regardless of daylight‑saving shifts or historic rule changes.

GLib.DateTime obtains its zone information from the system’s IANA tz database (/usr/share/zoneinfo), while GTK Calendar works with a plain GLib.Date and performs no automatic time‑zone adjustment. Developers must therefore call to_local() (or to_utc()) and extract the date, but the correctness of this step depends on the completeness and up‑to‑date nature of the zoneinfo files present on the target system. This raises portability concerns for applications that run on diverse Linux distributions or container images where the tz database may be outdated or missing.

1. Should developers bundle a known‑good tz file with their Vala application and load it via GLib.TimeZone.new() to guarantee consistent conversions?
2. Is there a recommended pattern for validating the round‑trip GLib.DateTime → GLib.Date → GLib.DateTime conversion at runtime to detect zone‑database mismatches?
3. How can an application safely handle dates that fall before the earliest rule defined in the host’s tz database without producing silent off‑by‑one errors?