Give the calendar one reading of a capture instant

The era-based conversion sat in library_ui.rs with two readers: the
timeline's month headings and, through format_date, the exporter's {date}
token. Import folder templates are a third, and three copies of a date
calculation that could drift apart is one too many — an image filed under a
date the timeline does not show it on is a file the user cannot find.

Moved to dr_types::time, which is where shared vocabulary lives, and given
the offset-aware reading an import needs: a shot taken at 23:30 in Tokyo
belongs in Tokyo's day, and filing by UTC would split one night's
photographs across two folders at whatever hour the offset happens to be.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-types/src/lib.rs

@@ -11,6 +11,7 @@ use std::ops::Range;
 pub mod colour;
 pub mod selector;
 pub mod settings;
+pub mod time;
 
 pub use colour::{Chromaticities, Transfer};
 pub use selector::{ColourLabel, DateSelector, FlagState, Selector, Tier};
@@ -18,6 +19,7 @@ pub use settings::{
     CacheSettings, CollisionPolicy, ColourSpace, DevelopSettings, ExportFormat, ExportSettings,
     ExportTarget, OutputSharpening, Settings, SizingMode,
 };
+pub use time::{civil_from_unix, civil_from_unix_at, format_date, Civil};
 
 /// Identifies a granted library location — a directory on Linux, a persisted
 /// document tree on Android.

core/dr-types/src/time.rs

@@ -0,0 +1,135 @@
+//! Capture instants as civil dates.
+//!
+//! Hand-rolled rather than pulling in chrono for four fields — the same
+//! reasoning as the connector's HTTP date parsing, and the same algorithm the
+//! timeline has always used.
+//!
+//! # Why this is shared vocabulary rather than a helper where it is needed
+//!
+//! Three places convert a capture instant to a date, and they must agree:
+//! the timeline's month headings, the exporter's `{date}` token (FR-EXP-6),
+//! and the folder an import writes into (FR-CAT-10, FR-NC-7a). A photograph
+//! filed under `2026-08-21` while the timeline shows it on the 22nd is not a
+//! cosmetic disagreement — it is a file the user cannot find where the app
+//! told them to look.
+
+/// A civil date and hour, as the calendar shows it.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Civil {
+    pub year: i64,
+    /// 1–12.
+    pub month: i64,
+    /// 1–31.
+    pub day: i64,
+    /// 0–23.
+    pub hour: i64,
+}
+
+/// Unix seconds to a civil date, via the usual era-based algorithm.
+///
+/// The reading is UTC. A capture instant carries its own zone separately
+/// (`Metadata::captured_offset`), because a photograph's date is local to
+/// where it was taken: see [`civil_from_unix_at`].
+pub fn civil_from_unix(t: i64) -> Civil {
+    let days = t.div_euclid(86_400);
+    let secs = t.rem_euclid(86_400);
+    let z = days + 719_468;
+    let era = if z >= 0 { z } else { z - 146_096 } / 146_097;
+    let doe = z - era * 146_097;
+    let yoe = (doe - doe / 1460 + doe / 36524 - doe / 146_096) / 365;
+    let y = yoe + era * 400;
+    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+    let mp = (5 * doy + 2) / 153;
+    let d = doy - (153 * mp + 2) / 5 + 1;
+    let m = if mp < 10 { mp + 3 } else { mp - 9 };
+    Civil {
+        year: if m <= 2 { y + 1 } else { y },
+        month: m,
+        day: d,
+        hour: secs / 3600,
+    }
+}
+
+/// The same instant read in the zone the camera recorded.
+///
+/// `offset` is minutes east of UTC, as EXIF `OffsetTimeOriginal` gives it.
+/// Absent on most bodies before ~2018, in which case the stored reading is
+/// already the local wall clock (see `Metadata::captured_at`) and passing
+/// `None` is correct rather than a fallback.
+///
+/// This is the reading an *import* must use. A shot taken at 23:30 in Tokyo
+/// belongs in Tokyo's day: filing it by UTC would put a night's photographs in
+/// two different folders, split at whatever hour the offset happens to be.
+pub fn civil_from_unix_at(t: i64, offset: Option<i32>) -> Civil {
+    civil_from_unix(t + offset.unwrap_or(0) as i64 * 60)
+}
+
+/// A capture instant as `YYYY-MM-DD`.
+pub fn format_date(t: i64) -> String {
+    let c = civil_from_unix(t);
+    format!("{}-{:02}-{:02}", c.year, c.month, c.day)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_epoch_is_the_first_of_january_1970() {
+        let c = civil_from_unix(0);
+        assert_eq!((c.year, c.month, c.day, c.hour), (1970, 1, 1, 0));
+    }
+
+    #[test]
+    fn a_leap_day_is_the_twenty_ninth_of_february() {
+        // 2024-02-29T12:00:00Z. A year-400 rule mistake shows up here first.
+        let c = civil_from_unix(1_709_208_000);
+        assert_eq!((c.year, c.month, c.day, c.hour), (2024, 2, 29, 12));
+    }
+
+    #[test]
+    fn the_year_2000_is_a_leap_year_and_1900_was_not() {
+        // 2000-02-29T00:00:00Z exists...
+        let c = civil_from_unix(951_782_400);
+        assert_eq!((c.year, c.month, c.day), (2000, 2, 29));
+        // ...and 1900-03-01 is the day after 1900-02-28, with no 29th between.
+        let feb28 = civil_from_unix(-2_203_977_600);
+        assert_eq!((feb28.year, feb28.month, feb28.day), (1900, 2, 28));
+        let next = civil_from_unix(-2_203_977_600 + 86_400);
+        assert_eq!((next.year, next.month, next.day), (1900, 3, 1));
+    }
+
+    #[test]
+    fn an_instant_before_the_epoch_does_not_wrap() {
+        // Euclidean division rather than truncating: a 1969 date must not
+        // round toward zero into 1970.
+        let c = civil_from_unix(-1);
+        assert_eq!((c.year, c.month, c.day, c.hour), (1969, 12, 31, 23));
+    }
+
+    #[test]
+    fn a_late_evening_shot_keeps_its_own_days_date() {
+        // 2026-08-21T22:30:00Z is already the 22nd in Tokyo (+540 minutes).
+        // Filing it by UTC would put it a day earlier than the photographer
+        // remembers taking it.
+        let utc = civil_from_unix_at(1_787_351_400, None);
+        assert_eq!((utc.year, utc.month, utc.day), (2026, 8, 21));
+        let tokyo = civil_from_unix_at(1_787_351_400, Some(540));
+        assert_eq!((tokyo.year, tokyo.month, tokyo.day), (2026, 8, 22));
+    }
+
+    #[test]
+    fn a_western_offset_can_move_the_date_back() {
+        // The mirror case: 2026-08-22T02:00:00Z is still the 21st in New York.
+        let ny = civil_from_unix_at(1_787_364_000, Some(-240));
+        assert_eq!((ny.year, ny.month, ny.day), (2026, 8, 21));
+    }
+
+    #[test]
+    fn a_formatted_date_is_zero_padded() {
+        assert_eq!(format_date(1_709_208_000), "2024-02-29");
+        // Single-digit months and days pad, so names sort lexicographically —
+        // which is the whole reason a folder template is worth having.
+        assert_eq!(format_date(0), "1970-01-01");
+    }
+}

ui/dr-ui/src/library_ui.rs

@@ -3428,27 +3428,18 @@ fn format_bucket(t: i64, g: dr_catalog::Granularity) -> String {
 /// reading so a filename and the timeline cannot disagree about what day a
 /// photograph was taken.
 pub fn format_date(t: i64) -> String {
-    let (y, m, d, _) = civil_from_unix(t);
+    dr_types::format_date(t)
-    format!("{y}-{m:02}-{d:02}")
 }
 
-/// Unix seconds to a civil date, via the usual era-based algorithm.
+/// Unix seconds to a civil date, in the tuple shape this module reads it in.
 ///
-/// Hand-rolled rather than pulling in chrono for four fields — the same
+/// The algorithm moved to `dr_types::time` when import folder templates
-/// reasoning as the connector's HTTP date parsing.
+/// (FR-CAT-10) became a third caller. Three readings of the same instant that
+/// could drift apart is one too many: an image filed under a date the timeline
+/// does not show it on is a file the user cannot find.
 fn civil_from_unix(t: i64) -> (i64, i64, i64, i64) {
-    let days = t.div_euclid(86_400);
+    let c = dr_types::civil_from_unix(t);
-    let secs = t.rem_euclid(86_400);
+    (c.year, c.month, c.day, c.hour)
-    let z = days + 719_468;
-    let era = if z >= 0 { z } else { z - 146_096 } / 146_097;
-    let doe = z - era * 146_097;
-    let yoe = (doe - doe / 1460 + doe / 36524 - doe / 146_096) / 365;
-    let y = yoe + era * 400;
-    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
-    let mp = (5 * doy + 2) / 153;
-    let d = doy - (153 * mp + 2) / 5 + 1;
-    let m = if mp < 10 { mp + 3 } else { mp - 9 };
-    (if m <= 2 { y + 1 } else { y }, m, d, secs / 3600)
 }
 
 /// Copy decoded RGBA into a Slint image.