diff --git a/contracts/split/src/lib.rs b/contracts/split/src/lib.rs
index f5e1624..fd53150 100644
--- a/contracts/split/src/lib.rs
+++ b/contracts/split/src/lib.rs
@@ -5405,7 +5405,7 @@ impl SplitContract {
             deadline > env.ledger().timestamp(),
             "deadline must be in the future"
         );
-        // Issue #430: creator-defined payment window.
+        // Issue #430 / #697: creator-defined payment window.
         if let Some(close_at) = payment_close_at {
             assert!(
                 close_at < deadline,
@@ -5413,10 +5413,9 @@ impl SplitContract {
             );
         }
         if let (Some(open_at), Some(close_at)) = (payment_open_at, payment_close_at) {
-            assert!(
-                open_at < close_at,
-                "payment_open_at must be before payment_close_at"
-            );
+            if open_at >= close_at {
+                env.panic_with_error(ContractError::InvalidAmount);
+            }
         }
         assert!(bonus_pool >= 0, "bonus_pool must be non-negative");
         // Issue #690: `penalty_bps` is a fraction of a late payment; a value
diff --git a/contracts/split/src/test.rs b/contracts/split/src/test.rs
index ca0e831..8dc6bbc 100644
--- a/contracts/split/src/test.rs
+++ b/contracts/split/src/test.rs
@@ -8495,3 +8495,88 @@ fn test_create_invoice_valid_multisig_setup_ok() {
     let id = c.create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts);
     assert!(id >= 1);
 }
+
+// --- Issue #697: payment_open_at must be strictly before payment_close_at when both are set ---
+
+#[test]
+fn test_create_invoice_payment_open_at_greater_than_close_at_rejected() {
+    let (env, contract_id, token_id) = setup_initialized();
+    let c = client(&env, &contract_id);
+    env.ledger().set_timestamp(50);
+
+    let creator = Address::generate(&env);
+    let (recipients, amounts) = two_recipients(&env);
+
+    let mut opts = default_options(&env);
+    opts.ext.payment_open_at = Some(100);
+    opts.ext.payment_close_at = Some(99);
+
+    let res = c.try_create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts);
+    assert_eq!(res, Err(Ok(ContractError::InvalidAmount.into())));
+}
+
+#[test]
+fn test_create_invoice_payment_open_at_equal_to_close_at_rejected() {
+    let (env, contract_id, token_id) = setup_initialized();
+    let c = client(&env, &contract_id);
+    env.ledger().set_timestamp(50);
+
+    let creator = Address::generate(&env);
+    let (recipients, amounts) = two_recipients(&env);
+
+    let mut opts = default_options(&env);
+    opts.ext.payment_open_at = Some(100);
+    opts.ext.payment_close_at = Some(100);
+
+    let res = c.try_create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts);
+    assert_eq!(res, Err(Ok(ContractError::InvalidAmount.into())));
+}
+
+#[test]
+fn test_create_invoice_payment_open_at_less_than_close_at_ok() {
+    let (env, contract_id, token_id) = setup_initialized();
+    let c = client(&env, &contract_id);
+    env.ledger().set_timestamp(50);
+
+    let creator = Address::generate(&env);
+    let (recipients, amounts) = two_recipients(&env);
+
+    let mut opts = default_options(&env);
+    opts.ext.payment_open_at = Some(99);
+    opts.ext.payment_close_at = Some(100);
+
+    let id = c.create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts);
+    assert!(id >= 1);
+}
+
+#[test]
+fn test_create_invoice_payment_window_only_one_or_none_ok() {
+    let (env, contract_id, token_id) = setup_initialized();
+    let c = client(&env, &contract_id);
+    env.ledger().set_timestamp(50);
+
+    let creator = Address::generate(&env);
+    let (recipients, amounts) = two_recipients(&env);
+
+    // Only payment_open_at set
+    let mut opts_open_only = default_options(&env);
+    opts_open_only.ext.payment_open_at = Some(100);
+    opts_open_only.ext.payment_close_at = None;
+    let id1 = c.create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts_open_only);
+    assert!(id1 >= 1);
+
+    // Only payment_close_at set
+    let mut opts_close_only = default_options(&env);
+    opts_close_only.ext.payment_open_at = None;
+    opts_close_only.ext.payment_close_at = Some(100);
+    let id2 = c.create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts_close_only);
+    assert!(id2 >= 1);
+
+    // Both None
+    let mut opts_none = default_options(&env);
+    opts_none.ext.payment_open_at = None;
+    opts_none.ext.payment_close_at = None;
+    let id3 = c.create_invoice(&creator, &recipients, &amounts, &token_id, &9_999_u64, &opts_none);
+    assert!(id3 >= 1);
+}
+