diff --git a/AGENTS.md b/AGENTS.md
index 43670025..316c267d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -113,6 +113,8 @@ only; English where there is none): never bundle a CJK font, they are 10 MB and
    GitHub release JSON fixtures (small, committed under `crates/core/tests/fixtures/`).
 5. When fetching anything, use a generic `ArtCraft-Launcher/<version>` User-Agent and never put
    a person's name, email or other personal details in requests.
+6. **Time goes through `jiff`** (workspace dependency). Never hand-roll UTC/calendar conversion
+   (days-to-date algorithms, leap-year math); a module owns only its output format string.
 
 ## 3. Before you finish a task
 
diff --git a/Cargo.lock b/Cargo.lock
index b381bfc1..e656d002 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -303,6 +303,7 @@ dependencies = [
 name = "artcraft-launcher-core"
 version = "0.6.1"
 dependencies = [
+ "jiff",
  "semver",
  "serde",
  "serde_json",
@@ -5875,6 +5876,9 @@ checksum = "ec7a2a501ed189703dba8b08142f057e887dfc4b2cc4db2d343ac6376ba3e0b9"
 [[package]]
 name = "xtask"
 version = "0.6.1"
+dependencies = [
+ "jiff",
+]
 
 [[package]]
 name = "yoke"
diff --git a/Cargo.toml b/Cargo.toml
index 673fdbac..805f2a7c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -28,6 +28,8 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 semver = { version = "1", features = ["serde"] }
 log = "0.4"
+# All date/time conversion goes through jiff (no hand-rolled calendar code; AGENTS.md).
+jiff = "0.2"
 
 [profile.dev.package."*"]
 opt-level = 2
diff --git a/crates/core/Cargo.toml b/crates/core/Cargo.toml
index 4806c1ce..c518d780 100644
--- a/crates/core/Cargo.toml
+++ b/crates/core/Cargo.toml
@@ -14,3 +14,5 @@ workspace = true
 serde = { workspace = true }
 serde_json = { workspace = true }
 semver = { workspace = true }
+# Calendar conversion (jiff); see AGENTS.md. Only civil-date math is used, no time zone.
+jiff = { workspace = true }
diff --git a/crates/core/src/time.rs b/crates/core/src/time.rs
index 22b580f4..34c19323 100644
--- a/crates/core/src/time.rs
+++ b/crates/core/src/time.rs
@@ -1,4 +1,5 @@
-//! Just enough calendar to read GitHub's timestamps and show them, without a date crate.
+//! Just enough calendar to read GitHub's timestamps and show them (the conversion goes through
+//! jiff; this module owns only the parsing and the output format strings).
 //!
 //! # Internationalisation
 //! [`format_ago`] and [`format_date`] are English-only fallbacks. Translatable UI must use
@@ -64,32 +65,25 @@ pub fn parse_rfc3339(text: &str) -> Option<i64> {
         }
         _ => return None,
     };
-    Some(days_from_civil(y, mo, d) * 86_400 + h * 3600 + mi * 60 + s - offset)
+    Some(days_from_civil(y, mo, d)? * 86_400 + h * 3600 + mi * 60 + s - offset)
 }
 
-/// Days since 1970-01-01 (Howard Hinnant's algorithm).
-fn days_from_civil(y: i64, m: i64, d: i64) -> i64 {
-    let y = if m <= 2 { y - 1 } else { y };
-    let era = y.div_euclid(400);
-    let yoe = y - era * 400;
-    let mp = (m + 9) % 12;
-    let doy = (153 * mp + 2) / 5 + d - 1;
-    let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
-    era * 146_097 + doe - 719_468
+/// Days since 1970-01-01 (through jiff's civil calendar). A day past the month's length rolls over
+/// the way the old Hinnant arithmetic did (e.g. `2026-02-30` → `2026-03-02`). `None` for a year
+/// outside jiff's range.
+fn days_from_civil(y: i64, m: i64, d: i64) -> Option<i64> {
+    let first = jiff::civil::Date::new(i16::try_from(y).ok()?, i8::try_from(m).ok()?, 1).ok()?;
+    let date = first.checked_add(jiff::SignedDuration::from_hours(d.checked_sub(1)?.checked_mul(24)?)).ok()?;
+    Some(date.duration_since(jiff::civil::Date::constant(1970, 1, 1)).as_secs() / 86_400)
 }
 
-/// (year, month, day) for a Unix time.
+/// (year, month, day) for a Unix time (UTC). A time outside jiff's calendar range reads as the
+/// epoch rather than panicking.
 pub fn civil_from_unix(secs: i64) -> (i64, u32, u32) {
-    let z = secs.div_euclid(86_400) + 719_468;
-    let era = z.div_euclid(146_097);
-    let doe = z - era * 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 m = if mp < 10 { mp + 3 } else { mp - 9 };
-    let y = yoe + era * 400 + i64::from(m <= 2);
-    (y, u32::try_from(m).unwrap_or(1), u32::try_from(d).unwrap_or(1))
+    let epoch = jiff::civil::Date::constant(1970, 1, 1);
+    let whole_days = jiff::SignedDuration::from_secs(secs.div_euclid(86_400).saturating_mul(86_400));
+    let date = epoch.checked_add(whole_days).unwrap_or(epoch);
+    (i64::from(date.year()), u32::try_from(date.month()).unwrap_or(1), u32::try_from(date.day()).unwrap_or(1))
 }
 
 const MONTHS: [&str; 12] = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
diff --git a/crates/ui-egui/Cargo.toml b/crates/ui-egui/Cargo.toml
index 235558cd..aebc6962 100644
--- a/crates/ui-egui/Cargo.toml
+++ b/crates/ui-egui/Cargo.toml
@@ -24,7 +24,7 @@ sys-locale = "0.3"
 # Finding the Simplified Chinese face in a system font collection; egui already uses it.
 skrifa = { version = "0.44", default-features = false, features = ["std"] }
 # The system time zone, so dates read as this computer's calendar; already in the desktop build.
-jiff = "0.2"
+jiff = { workspace = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 # The browser's time zone, for the same dates.
diff --git a/xtask/Cargo.toml b/xtask/Cargo.toml
index f027cec0..941b3d56 100644
--- a/xtask/Cargo.toml
+++ b/xtask/Cargo.toml
@@ -7,3 +7,6 @@ publish = false
 
 [lints]
 workspace = true
+
+[dependencies]
+jiff = { workspace = true }
diff --git a/xtask/src/local.rs b/xtask/src/local.rs
index f818d21d..eae1fea3 100644
--- a/xtask/src/local.rs
+++ b/xtask/src/local.rs
@@ -493,20 +493,13 @@ fn unix_now() -> i64 {
     SystemTime::now().duration_since(UNIX_EPOCH).map_or(0, |d| i64::try_from(d.as_secs()).unwrap_or(0))
 }
 
-/// `2026-10-10T12:00:00Z` (civil-from-days, Howard Hinnant's algorithm).
+/// `2026-10-10T12:00:00Z` (through jiff's civil calendar).
 fn rfc3339(secs: i64) -> String {
-    let days = secs.div_euclid(86_400);
     let rem = secs.rem_euclid(86_400);
-    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 m = if mp < 10 { mp + 3 } else { mp - 9 };
-    let y = yoe + era * 400 + i64::from(m <= 2);
-    format!("{y:04}-{m:02}-{d:02}T{:02}:{:02}:{:02}Z", rem / 3600, rem % 3600 / 60, rem % 60)
+    let epoch = jiff::civil::Date::constant(1970, 1, 1);
+    let whole_days = jiff::SignedDuration::from_secs(secs.div_euclid(86_400).saturating_mul(86_400));
+    let date = epoch.checked_add(whole_days).unwrap_or(epoch);
+    format!("{:04}-{:02}-{:02}T{:02}:{:02}:{:02}Z", date.year(), date.month(), date.day(), rem / 3600, rem % 3600 / 60, rem % 60)
 }
 
 // ---- versions -----------------------------------------------------------------------------------