Use chrono for UTC/calendar conversion instead of hand-rolled copies

Replaces the two hand-rolled Howard Hinnant days-from-civil copies with chrono
as a workspace dependency:
- crates/collab/src/message.rs: utc_date (YYYY-MM-DD HH:MM UTC)
- crates/ui-egui/src/home_recents.rs: date (Mon D, YYYY, UTC; keeps the
  translated month names and trf formatting)
Output is unchanged. Neither function had a test, so each gains one pinning the
exact strings the old code produced (leap days included). chrono uses only std,
so wasm stays safe. AGENTS.md/CLAUDE.md now say time goes through chrono.

Follow-up to storytold/cadcraft#1785

AGENTS.md

@@ -81,6 +81,7 @@ code.
   L6 depends on egui/eframe/rfd/winit. **The UI crate is swappable.**
 - **The UI is thin**: panels read engine state and act through `app.run(id, params)` /
   `app.set_props(ids, props)`. Colours come from `theme::Tokens`.
+- **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string.
 - **Rust only** (no handwritten JS/TS). **Never break wasm** (`cargo xtask wasm`).
 
 ### Quality gates (before every commit)

CLAUDE.md

@@ -81,6 +81,7 @@ code.
   L6 depends on egui/eframe/rfd/winit. **The UI crate is swappable.**
 - **The UI is thin**: panels read engine state and act through `app.run(id, params)` /
   `app.set_props(ids, props)`. Colours come from `theme::Tokens`.
+- **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string.
 - **Rust only** (no handwritten JS/TS). **Never break wasm** (`cargo xtask wasm`).
 
 ### Quality gates (before every commit)

Cargo.lock

@@ -742,6 +742,15 @@ dependencies = [
  "rand_core 0.10.1",
 ]
 
+[[package]]
+name = "chrono"
+version = "0.4.45"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1aa79e62e7697b8e29b513a68abacf485adcd1fe8284a4316c5ae868e6633327"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "clipboard-win"
 version = "5.4.1"
@@ -1499,6 +1508,7 @@ name = "figcraft-collab"
 version = "0.9.0"
 dependencies = [
  "base64 0.22.1",
+ "chrono",
  "figcraft-color",
  "figcraft-doc",
  "figcraft-geom",
@@ -1744,6 +1754,7 @@ name = "figcraft-ui-egui"
 version = "0.9.0"
 dependencies = [
  "base64 0.22.1",
+ "chrono",
  "egui",
  "egui_extras",
  "figcraft-color",

Cargo.toml

@@ -45,6 +45,8 @@ serde = { version = "1", features = ["derive", "rc"] }
 serde_json = { version = "1", features = ["float_roundtrip"] }
 regex = "1.13"
 log = "0.4"
+# All date/time conversion goes through chrono (no hand-rolled calendar code; AGENTS.md).
+chrono = { version = "0.4.45", default-features = false, features = ["std"] }
 proptest = "1"
 imbl = { version = "7", features = ["serde"] }
 postcard = { version = "1", default-features = false, features = ["alloc"] }

crates/collab/Cargo.toml

@@ -13,6 +13,7 @@ figcraft-doc = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
+chrono = { workspace = true }
 imbl = { workspace = true }
 base64 = { workspace = true }
 postcard = { workspace = true }

crates/collab/src/message.rs

@@ -699,19 +699,8 @@ pub enum ServerMsg {
 /// `YYYY-MM-DD HH:MM UTC` for seconds since 1970 (how servers and apps show the times in these
 /// messages).
 pub fn utc_date(secs: u64) -> String {
-    let days = i64::try_from(secs / 86_400).unwrap_or(0);
-    let (h, m) = ((secs % 86_400) / 3600, (secs % 3600) / 60);
-    // Days to a civil date (proleptic Gregorian).
-    let z = days + 719_468;
-    let era = z.div_euclid(146_097);
-    let doe = z.rem_euclid(146_097);
-    let yoe = (doe - doe / 1460 + doe / 36_524 - doe / 146_096) / 365;
-    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
-    let mp = (5 * doy + 2) / 153;
-    let d = doy - (153 * mp + 2) / 5 + 1;
-    let mo = if mp < 10 { mp + 3 } else { mp - 9 };
-    let y = yoe + era * 400 + i64::from(mo <= 2);
-    format!("{y:04}-{mo:02}-{d:02} {h:02}:{m:02} UTC")
+    let t = i64::try_from(secs).ok().and_then(|s| chrono::DateTime::from_timestamp(s, 0)).unwrap_or_default();
+    t.format("%Y-%m-%d %H:%M UTC").to_string()
 }
 
 /// The binary form of `msg`: [`WIRE_VERSION`], then postcard.
@@ -752,3 +741,19 @@ pub fn decode_json<T: DeserializeOwned>(text: &str, max_bytes: usize) -> Result<
     }
     serde_json::from_str(text).map_err(|e| Error::Decode(e.to_string()))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// `utc_date` keeps the exact `YYYY-MM-DD HH:MM UTC` output the hand-rolled civil-from-days
+    /// code produced (leap days and all).
+    #[test]
+    fn utc_date_formats_seconds_since_1970() {
+        assert_eq!(utc_date(0), "1970-01-01 00:00 UTC");
+        assert_eq!(utc_date(1_000_000), "1970-01-12 13:46 UTC");
+        assert_eq!(utc_date(951_782_400), "2000-02-29 00:00 UTC");
+        assert_eq!(utc_date(1_709_164_800), "2024-02-29 00:00 UTC");
+        assert_eq!(utc_date(1_791_208_980), "2026-10-05 14:03 UTC");
+    }
+}

crates/ui-egui/Cargo.toml

@@ -22,6 +22,7 @@ egui = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 log = { workspace = true }
+chrono = { workspace = true }
 base64 = { workspace = true }
 image = { workspace = true }
 egui_extras = { workspace = true }

crates/ui-egui/src/home_recents.rs

@@ -10,6 +10,7 @@
 use std::collections::HashMap;
 use std::path::Path;
 
+use chrono::Datelike;
 use egui::{Align2, Color32, CornerRadius, Id, Key, Rect, Response, Sense, Stroke, StrokeKind, Ui, pos2, vec2};
 use figcraft_engine::Prefs;
 use figcraft_format::thumbnail::{FileKind, Preview};
@@ -202,16 +203,8 @@ const MONTHS: [&str; 12] = [
 
 /// `secs` since 1970 as a date in UTC: "Mar 4, 2026".
 fn date(secs: u64) -> String {
-    // Days to a civil date (Howard Hinnant's days_from_civil, inverted).
-    let z = i64::try_from(secs / 86_400).unwrap_or(0) + 719_468;
-    let era = z.div_euclid(146_097);
-    let doe = z.rem_euclid(146_097);
-    let yoe = (doe - doe / 1460 + doe / 36_524 - doe / 146_096) / 365;
-    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
-    let mp = (5 * doy + 2) / 153;
-    let day = doy - (153 * mp + 2) / 5 + 1;
-    let month = if mp < 10 { mp + 3 } else { mp - 9 };
-    let year = yoe + era * 400 + i64::from(month <= 2);
+    let dt = i64::try_from(secs).ok().and_then(|s| chrono::DateTime::from_timestamp(s, 0)).unwrap_or_default().naive_utc();
+    let (year, month, day) = (i64::from(dt.year()), i64::from(dt.month()), i64::from(dt.day()));
     let name = usize::try_from(month - 1).ok().and_then(|m| MONTHS.get(m)).map_or("", |m| tr(m));
     trf("{month} {day}, {year}", &[("month", &name), ("day", &day), ("year", &year)])
 }
@@ -965,3 +958,18 @@ fn list(ui: &mut Ui, t: &Tokens, recents: &mut Recents, files: &[Shown], now: u6
     arrows(ui, recents, files, 0, 1);
     action
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// `date` keeps the exact "Mon D, YYYY" (UTC) output the hand-rolled days-from-civil code
+    /// produced. With no catalog loaded the month and format strings pass through in English.
+    #[test]
+    fn date_formats_seconds_since_1970_in_utc() {
+        assert_eq!(date(0), "Jan 1, 1970");
+        assert_eq!(date(951_782_400), "Feb 29, 2000");
+        assert_eq!(date(1_709_164_800), "Feb 29, 2024");
+        assert_eq!(date(1_741_046_400), "Mar 4, 2025");
+    }
+}