diff --git a/backend/src/services/indexer.ts b/backend/src/services/indexer.ts
index 0e0914ec..ec4b65f0 100644
--- a/backend/src/services/indexer.ts
+++ b/backend/src/services/indexer.ts
@@ -297,6 +297,22 @@ function processEvent(db: any, event: rpc.Api.EventResponse): void {
           event.ledger,
         );
         break;
+
+      case "StartUp":
+        recordEventWithDb(
+          db,
+          value.stream_id.toString(),
+          "start_time_updated",
+          timestamp,
+          value.sender,
+          undefined,
+          {
+            oldStartTime: Number(value.old_start_time),
+            newStartTime: Number(value.new_start_time),
+          },
+          event.ledger,
+        );
+        break;
     }
   } catch (err) {
     logger.error({ err }, "failed to process event");
diff --git a/contracts/src/lib.rs b/contracts/src/lib.rs
index 82777616..2fff608f 100644
--- a/contracts/src/lib.rs
+++ b/contracts/src/lib.rs
@@ -96,6 +96,16 @@ pub struct StreamResumed {
 
 }
 
+#[contracttype]
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct StreamStartTimeUpdated {
+    pub stream_id: u64,
+    pub sender: Address,
+    pub old_start_time: u64,
+    pub new_start_time: u64,
+    pub updated_at: u64,
+}
+
 #[contracttype]
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct ClawbackExecuted {
@@ -589,6 +599,55 @@ impl StellarStreamContract {
         );
     }
 
+    // ---------------------------------------------------------------------------
+    // Update start time
+    // ---------------------------------------------------------------------------
+
+    /// Update the start time of a stream.
+    ///
+    /// Only the original sender may invoke this function. The new start time
+    /// must be strictly in the future relative to the current ledger time,
+    /// strictly less than the stream's end time, and the stream must not be
+    /// canceled. Emits a `Stream/StartUp` event so off-chain listeners can
+    /// reconcile.
+    pub fn update_start_time(env: Env, stream_id: u64, sender: Address, new_start_time: u64) {
+        let mut stream = read_stream(&env, stream_id);
+        if stream.sender != sender {
+            panic!("sender mismatch");
+        }
+        sender.require_auth();
+
+        if stream.canceled {
+            panic!("stream canceled");
+        }
+
+        let now = env.ledger().timestamp();
+        if new_start_time <= now {
+            panic!("new_start_time must be in the future");
+        }
+        if new_start_time >= stream.end_time {
+            panic!("new_start_time must be before stream end_time");
+        }
+
+        let old_start_time = stream.start_time;
+        stream.start_time = new_start_time;
+
+        env.storage()
+            .persistent()
+            .set(&DataKey::Stream(stream_id), &stream);
+
+        env.events().publish(
+            (symbol_short!("Stream"), symbol_short!("StartUp")),
+            StreamStartTimeUpdated {
+                stream_id,
+                sender,
+                old_start_time,
+                new_start_time,
+                updated_at: now,
+            },
+        );
+    }
+
     // -----------------------------------------------------------------------
     // Clawback
     // -----------------------------------------------------------------------
diff --git a/contracts/src/test.rs b/contracts/src/test.rs
index 7807efed..11295bd5 100644
--- a/contracts/src/test.rs
+++ b/contracts/src/test.rs
@@ -2332,3 +2332,173 @@ fn test_cancel_after_partial_claim_full_lifecycle() {
     let recipient_balance = token_client.balance(&recipient);
     assert_eq!(sender_refund + recipient_balance, 100);
 }
+
+// ============================================================================
+// #589 \u2014 update_start_time tests
+// ============================================================================
+
+#[test]
+fn test_update_start_time_success() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    // Create a stream scheduled to start in the future.
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    let stream_before = client.get_stream(&stream_id);
+    assert_eq!(stream_before.start_time, 2000);
+
+    // Move the ledger forward so new_start_time (2500) is in the future.
+    env.ledger().with_mut(|l| l.timestamp = 1000);
+
+    // Update the start time to a new future time before end_time (3000).
+    client.update_start_time(&stream_id, &sender, &2500);
+
+    let stream_after = client.get_stream(&stream_id);
+    assert_eq!(stream_after.start_time, 2500);
+}
+
+#[test]
+fn test_update_start_time_emits_event() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    env.ledger().with_mut(|l| l.timestamp = 1000);
+    client.update_start_time(&stream_id, &sender, &2500);
+
+    let last_event = env.events().all().last().unwrap();
+    assert_eq!(last_event.0, contract_id);
+    assert_eq!(
+        last_event.1,
+        (symbol_short!("Stream"), symbol_short!("StartUp")).into_val(&env)
+    );
+
+    let event_data: StreamStartTimeUpdated = last_event.2.into_val(&env);
+    assert_eq!(event_data.stream_id, stream_id);
+    assert_eq!(event_data.sender, sender);
+    assert_eq!(event_data.old_start_time, 2000);
+    assert_eq!(event_data.new_start_time, 2500);
+    assert_eq!(event_data.updated_at, 1000);
+}
+
+#[test]
+#[should_panic(expected = "sender mismatch")]
+fn test_update_start_time_fails_with_wrong_sender() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let wrong_sender = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    env.ledger().with_mut(|l| l.timestamp = 1000);
+    client.update_start_time(&stream_id, &wrong_sender, &2500);
+}
+
+#[test]
+#[should_panic(expected = "new_start_time must be in the future")]
+fn test_update_start_time_rejects_past_timestamp() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    env.ledger().with_mut(|l| l.timestamp = 2500);
+    // 2000 is not strictly greater than now (2500)
+    client.update_start_time(&stream_id, &sender, &2000);
+}
+
+#[test]
+#[should_panic(expected = "new_start_time must be before stream end_time")]
+fn test_update_start_time_rejects_at_end_time() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    env.ledger().with_mut(|l| l.timestamp = 1000);
+    // Setting new_start_time equal to end_time (3000) must be rejected.
+    client.update_start_time(&stream_id, &sender, &3000);
+}
+
+#[test]
+#[should_panic(expected = "new_start_time must be before stream end_time")]
+fn test_update_start_time_rejects_after_end_time() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    env.ledger().with_mut(|l| l.timestamp = 1000);
+    client.update_start_time(&stream_id, &sender, &3500);
+}
+
+#[test]
+#[should_panic(expected = "stream canceled")]
+fn test_update_start_time_rejects_on_canceled_stream() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let contract_id = env.register_contract(None, StellarStreamContract);
+    let client = StellarStreamContractClient::new(&env, &contract_id);
+
+    let admin = Address::generate(&env);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+    let token = create_token(&env, &admin);
+    let token_admin = token::StellarAssetClient::new(&env, &token);
+    token_admin.mint(&sender, &1000);
+
+    let stream_id = client.create_stream(&sender, &recipient, &token, &1000, &2000, &3000, &0, &None);
+    client.cancel(&stream_id, &sender);
+    env.ledger().with_mut(|l| l.timestamp = 500);
+    client.update_start_time(&stream_id, &sender, &2500);
+}