max / goingson

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1	//! System time-zone resolution for recurrence.
2	//!
3	//! GoingsOn stores instants in UTC and has no per-user time-zone preference, so
4	//! "the user's zone" is the operating system's zone. Recurrence advances dates in
5	//! this zone (see `goingson_core::recurrence`) so a fixed local time-of-day, "every
6	//! day at 09:00", survives daylight-saving transitions instead of drifting an hour.
7
8	use chrono::{DateTime, NaiveDateTime, TimeZone, Utc};
9	use chrono_tz::Tz;
10
11	/// The user's IANA time zone, resolved from the OS. Falls back to UTC if the OS
12	/// zone can't be read or doesn't parse as a known IANA name.
13	pub fn system_tz() -> Tz {
14	iana_time_zone::get_timezone()
15	.ok()
16	.and_then(\|name\| name.parse().ok())
17	.unwrap_or(Tz::UTC)
18	}
19
20	/// Interpret a civil (wall-clock) datetime as being in the user's system zone
21	/// and convert it to the corresponding UTC instant.
22	///
23	/// Window queries (weekly/monthly review) work with civil dates like "the start
24	/// of this week"; those midnights are local, not UTC, so stamping them as UTC
25	/// misattributes edge-of-period rows by the zone offset. On a DST spring-forward
26	/// gap the civil time doesn't exist, so fall back to treating it as UTC rather
27	/// than panicking.
28	pub fn local_civil_to_utc(civil: NaiveDateTime) -> DateTime<Utc> {
29	system_tz()
30	.from_local_datetime(&civil)
31	.earliest()
32	.map_or_else(
33	\|\| DateTime::<Utc>::from_naive_utc_and_offset(civil, Utc),
34	\|dt\| dt.with_timezone(&Utc),
35	)
36	}
37