diff --git a/crates/zx_document_core/src/lib.rs b/crates/zx_document_core/src/lib.rs
index d0abe52..ea005eb 100644
--- a/crates/zx_document_core/src/lib.rs
+++ b/crates/zx_document_core/src/lib.rs
@@ -16,3 +16,4 @@ pub mod reading_material;
 pub mod search;
 pub mod session_v2;
 pub mod text;
+pub mod time_tracker;
diff --git a/crates/zx_document_core/src/time_tracker.rs b/crates/zx_document_core/src/time_tracker.rs
new file mode 100644
index 0000000..6fc3a66
--- /dev/null
+++ b/crates/zx_document_core/src/time_tracker.rs
@@ -0,0 +1,166 @@
+/// Tracks active reading time and calculates delta seconds.
+///
+/// iOS controls when to call tick (every 15s), pause (background), resume (foreground).
+/// Rust does NOT create its own timer.
+#[derive(Debug, Clone)]
+pub struct ActiveTimeTracker {
+    last_tick_ms: Option<i64>,
+    is_active: bool,
+    remainder_ms: i64,
+}
+
+impl ActiveTimeTracker {
+    pub fn new() -> Self {
+        Self {
+            last_tick_ms: None,
+            is_active: false,
+            remainder_ms: 0,
+        }
+    }
+
+    /// Start tracking. Returns delta = 0.
+    pub fn start(&mut self, timestamp_ms: i64) -> u32 {
+        self.last_tick_ms = Some(timestamp_ms);
+        self.is_active = true;
+        self.remainder_ms = 0;
+        0
+    }
+
+    /// Pause tracking. Returns accumulated delta since last tick.
+    pub fn pause(&mut self, timestamp_ms: i64) -> u32 {
+        let delta = self.tick(timestamp_ms);
+        self.is_active = false;
+        delta
+    }
+
+    /// Resume after pause. No delta produced.
+    pub fn resume(&mut self, timestamp_ms: i64) {
+        self.is_active = true;
+        self.last_tick_ms = Some(timestamp_ms);
+    }
+
+    /// Regular tick. Returns delta seconds since last tick (or pause).
+    /// Returns 0 if not active or time goes backwards.
+    pub fn tick(&mut self, timestamp_ms: i64) -> u32 {
+        if !self.is_active {
+            return 0;
+        }
+        let last = match self.last_tick_ms {
+            Some(t) => t,
+            None => {
+                self.last_tick_ms = Some(timestamp_ms);
+                return 0;
+            }
+        };
+        if timestamp_ms <= last {
+            return 0;
+        }
+        let elapsed_ms = timestamp_ms - last;
+        let total_ms = elapsed_ms + self.remainder_ms;
+        let delta_seconds = (total_ms / 1000) as u32;
+        self.remainder_ms = total_ms % 1000;
+        self.last_tick_ms = Some(timestamp_ms);
+        delta_seconds
+    }
+
+    /// Close tracking. Returns final residual delta.
+    pub fn close(&mut self, timestamp_ms: i64) -> u32 {
+        let delta = self.tick(timestamp_ms);
+        self.is_active = false;
+        delta
+    }
+
+    pub fn is_active(&self) -> bool {
+        self.is_active
+    }
+}
+
+impl Default for ActiveTimeTracker {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_start_returns_zero() {
+        let mut t = ActiveTimeTracker::new();
+        assert_eq!(t.start(1000), 0);
+        assert!(t.is_active());
+    }
+
+    #[test]
+    fn test_43_seconds_total() {
+        let mut t = ActiveTimeTracker::new();
+        t.start(0);
+        let d1 = t.tick(15_000);  // 15s
+        let d2 = t.tick(30_000);  // 15s
+        let d3 = t.close(43_000); // 13s residual
+        assert_eq!(d1, 15);
+        assert_eq!(d2, 15);
+        assert_eq!(d3, 13);
+        assert_eq!(d1 + d2 + d3, 43);
+    }
+
+    #[test]
+    fn test_pause_stops_time() {
+        let mut t = ActiveTimeTracker::new();
+        t.start(0);
+        t.tick(15_000);  // 15s
+        t.pause(15_000);
+        t.resume(45_000); // 30s pause
+        let d = t.tick(60_000); // 15s after resume
+        assert_eq!(d, 15); // only 15s counted, not 45s
+    }
+
+    #[test]
+    fn test_time_goes_backwards() {
+        let mut t = ActiveTimeTracker::new();
+        t.start(10000);
+        assert_eq!(t.tick(5000), 0); // backwards
+        assert_eq!(t.tick(25000), 15); // normal after backwards reset
+    }
+
+    #[test]
+    fn test_inactive_tick_returns_zero() {
+        let mut t = ActiveTimeTracker::new();
+        assert_eq!(t.tick(5000), 0); // not started
+        t.start(0);
+        t.pause(5000);
+        assert_eq!(t.tick(10000), 0); // paused
+    }
+
+    #[test]
+    fn test_close_returns_residual() {
+        let mut t = ActiveTimeTracker::new();
+        t.start(0);
+        t.tick(15000);
+        let d = t.close(20500); // 5.5s → 5s, 500ms remainder
+        assert_eq!(d, 5);
+        assert!(!t.is_active());
+    }
+
+    #[test]
+    fn test_remainder_accumulates() {
+        let mut t = ActiveTimeTracker::new();
+        t.start(0);
+        // Each tick: 1500ms → 1s + 500ms remainder
+        let d1 = t.tick(1500);
+        let d2 = t.tick(3000);
+        let d3 = t.tick(4500);
+        // Total: 3 ticks → d1=1, d2=2(with remainder), d3=1
+        // 1500ms = 1s, remainder 500ms
+        // tick2: 1500ms + 500ms remainder = 2000ms = 2s
+        // tick3: 1500ms = 1s, remainder 500ms
+        assert_eq!(d1 + d2 + d3, 4); // 1 + 2 + 1 = 4
+    }
+
+    #[test]
+    fn test_close_inactive_returns_zero() {
+        let mut t = ActiveTimeTracker::new();
+        assert_eq!(t.close(1000), 0);
+    }
+}