diff --git a/docs_v1.0/DESIGN/FILE_LIFECYCLE_V1.0.md b/docs_v1.0/DESIGN/FILE_LIFECYCLE_V1.0.md
index 5abf78d..d77d876 100644
--- a/docs_v1.0/DESIGN/FILE_LIFECYCLE_V1.0.md
+++ b/docs_v1.0/DESIGN/FILE_LIFECYCLE_V1.0.md
@@ -23,8 +23,8 @@ status: "draft"
 
 ```
 SHA256 = DNA or fingerprint (immutable biometric identity)
-file created time = birth moment
-birthday (UUID anchor) = birth timestamp
+file mtime = birth moment (preserved by rsync across systems)
+birthday (UUID anchor) = mtime timestamp
 .pre.json = birth certificate
 POST /api/v1/files/register = civil registration
 status = registered = citizenship completed
@@ -48,8 +48,8 @@ File watcher (`src/watcher/watcher.rs`) polls monitored directories every 60 sec
 ### Computation Steps
 
 ```
-1. fs::metadata(path).created()
-   → birthday = file creation time (RFC 3339)
+1. fs::metadata(path).modified()
+   → birthday = file modification time (mtime, RFC 3339; preserved by rsync -a across systems)
 
 2. SHA256(full file, streaming 64KB chunks)
    → content_hash = 512-bit hex string (file DNA / fingerprint)
@@ -94,7 +94,7 @@ Stored alongside other processor outputs:
 
 ### Key Design: UUID = f(mac, birthday, path, filename)
 
-The `birthday` is `file created time` — obtained from `fs::metadata().created()`. This is the **true birth time** of the file, not the registration time.
+The `birthday` is `file modification time` (mtime) — obtained from `fs::metadata().modified()`. Using mtime instead of birthtime ensures UUID stability when files are transferred between systems via rsync (which preserves mtime but not birthtime on macOS).
 
 ```
 birthday = 2026-05-15T02:15:00Z    ← file birth time, never changes
@@ -144,7 +144,7 @@ curl -X POST http://localhost:3002/api/v1/files/register \
 
 | Field | Source | Computed When | Mutable |
 |-------|--------|---------------|:------:|
-| `birthday` | `fs::metadata().created()` | Pre-process (once) | ❌ Never |
+| `birthday` | `fs::metadata().modified()` (mtime) | Pre-process (once) | ❌ Never (stable across rsync) |
 | `content_hash` (SHA256) | Full file | Pre-process (once) | ❌ Never (unless file modified) |
 | `file_uuid` | SHA256(mac\|birthday\|path\|filename) | Pre-process (once) | ❌ Never |
 | `registration_time` | `NOW()` at register | Register API | ✅ Per registration |
diff --git a/src/api/server.rs b/src/api/server.rs
index 60349bb..32eeb49 100644
--- a/src/api/server.rs
+++ b/src/api/server.rs
@@ -922,7 +922,7 @@ async fn register_single_file(
 
     let birthday = std::fs::metadata(&path)
         .ok()
-        .and_then(|m| m.created().ok())
+        .and_then(|m| m.modified().ok())
         .map(|t| {
             let secs = t.duration_since(std::time::UNIX_EPOCH).unwrap_or_default().as_secs();
             chrono::DateTime::from_timestamp(secs as i64, 0)
diff --git a/src/watcher/watcher.rs b/src/watcher/watcher.rs
index 47a106b..809f97c 100644
--- a/src/watcher/watcher.rs
+++ b/src/watcher/watcher.rs
@@ -93,7 +93,7 @@ pub async fn pre_process_file(file_path: &str) -> Option<String> {
         .unwrap_or_else(|_| "/Users/accusys/momentry/output_dev".to_string());
 
     let birthday = std::fs::metadata(&path).ok()
-        .and_then(|m| m.created().ok())
+        .and_then(|m| m.modified().ok())
         .map(|t| {
             let secs = t.duration_since(std::time::UNIX_EPOCH).unwrap_or_default().as_secs();
             chrono::DateTime::from_timestamp(secs as i64, 0)