diff --git a/.cspell/custom-words.txt b/.cspell/custom-words.txt
index ce73c361..6c36be88 100644
--- a/.cspell/custom-words.txt
+++ b/.cspell/custom-words.txt
@@ -118,6 +118,7 @@ okhttp
 opentelemetry
 otelgrpc
 otelhttp
+otherpisp
 Otherville
 OURCYGPATTERN
 Palo
@@ -127,6 +128,10 @@ Payoneer
 paypal
 Payplug
 pids
+PISP
+Pisp
+Pisps
+pisp
 pmezard
 proguard
 Proguard
@@ -149,6 +154,7 @@ ropeproject
 RPCURL
 Rulebook
 screenreaders
+SECP
 setlocal
 sharedpref
 Shopcider
diff --git a/code/sdk/python/ap2/sdk/constraints.py b/code/sdk/python/ap2/sdk/constraints.py
index 48aed798..3989f814 100644
--- a/code/sdk/python/ap2/sdk/constraints.py
+++ b/code/sdk/python/ap2/sdk/constraints.py
@@ -8,7 +8,7 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from datetime import datetime, timedelta
+from datetime import UTC, datetime, timedelta
 
 from ap2.sdk.generated.open_checkout_mandate import (
     AllowedMerchants,
@@ -316,8 +316,13 @@ def evaluate(
 class ExecutionDateEvaluator(PaymentConstraintEvaluator):
     """Evaluates if the execution date is within the allowed window."""
 
-    def __init__(self, constraint: ExecutionDate):
+    def __init__(
+        self,
+        constraint: ExecutionDate,
+        current_time: datetime | None = None,
+    ):
         self.constraint = constraint
+        self.current_time = current_time
 
     def evaluate(
         self,
@@ -326,7 +331,12 @@ def evaluate(
     ) -> list[str]:
         exec_date = closed_mandate.execution_date
         if not exec_date:
-            return []
+            effective_time = self.current_time or datetime.now(UTC)
+            if effective_time.tzinfo is None:
+                effective_time = effective_time.replace(tzinfo=UTC)
+            exec_date = effective_time.astimezone(UTC).isoformat().replace(
+                '+00:00', 'Z'
+            )
 
         violations = []
         if (
@@ -357,6 +367,7 @@ def create_payment_evaluator(  # noqa: PLR0911
         | PaymentReference
     ),
     mandate_context: MandateContext | None = None,
+    current_time: datetime | None = None,
 ) -> PaymentConstraintEvaluator:
     """Factory: create the appropriate evaluator for a payment constraint."""
     if isinstance(constraint, AmountRange):
@@ -374,7 +385,7 @@ def create_payment_evaluator(  # noqa: PLR0911
     if isinstance(constraint, Budget):
         return BudgetEvaluator(constraint, mandate_context)
     if isinstance(constraint, ExecutionDate):
-        return ExecutionDateEvaluator(constraint)
+        return ExecutionDateEvaluator(constraint, current_time)
     raise ValueError(f'Unknown payment constraint type: {type(constraint)}')
 
 
@@ -491,6 +502,7 @@ def check_payment_constraints(
     closed_payment: PaymentMandate,
     open_checkout_hash: str | None = None,
     mandate_context: MandateContext | None = None,
+    current_time: datetime | None = None,
 ) -> list[str]:
     """Verify the closed payment satisfies open mandate constraints.
 
@@ -503,6 +515,7 @@ def check_payment_constraints(
       open_checkout_hash: The hash of the open checkout mandate, required for
         `PaymentReference` constraints.
       mandate_context: Aggregated usage context for the mandate.
+      current_time: Trusted current time used for immediate payments.
 
     Returns:
       A list of strings, where each string describes a violation of the
@@ -533,7 +546,11 @@ def check_payment_constraints(
             )
 
     for constraint in open_mandate.constraints:
-        evaluator = create_payment_evaluator(constraint, mandate_context)
+        evaluator = create_payment_evaluator(
+            constraint,
+            mandate_context,
+            current_time,
+        )
         violations.extend(
             evaluator.evaluate(closed_payment, open_checkout_hash)
         )
diff --git a/code/sdk/python/ap2/sdk/payment_mandate_chain.py b/code/sdk/python/ap2/sdk/payment_mandate_chain.py
index eec04a65..617ee286 100644
--- a/code/sdk/python/ap2/sdk/payment_mandate_chain.py
+++ b/code/sdk/python/ap2/sdk/payment_mandate_chain.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
+from datetime import datetime
 from typing import Any
 
 from ap2.sdk.constraints import MandateContext, check_payment_constraints
@@ -40,6 +41,7 @@ def verify(
         expected_transaction_id: str | None = None,
         expected_open_checkout_hash: str | None = None,
         mandate_context: MandateContext | None = None,
+        current_time: datetime | None = None,
     ) -> list[str]:
         """Verifies the constraints of the payment mandate chain.
 
@@ -49,6 +51,7 @@ def verify(
           expected_open_checkout_hash: Optional checkout hash to check against
             the open mandate's checkout_reference.
           mandate_context: Aggregated usage context for the mandate.
+          current_time: Trusted current time used for immediate payments.
 
         Returns:
           A list of strings describing any violations found.
@@ -71,6 +74,7 @@ def verify(
             self.closed_mandate,
             open_checkout_hash=expected_open_checkout_hash,
             mandate_context=mandate_context,
+            current_time=current_time,
         )
         if (
             expected_transaction_id is not None
diff --git a/code/sdk/python/ap2/tests/constraints_tests.py b/code/sdk/python/ap2/tests/constraints_tests.py
index 910c97fc..2b09bb6a 100644
--- a/code/sdk/python/ap2/tests/constraints_tests.py
+++ b/code/sdk/python/ap2/tests/constraints_tests.py
@@ -1,6 +1,7 @@
 """Tests for centralized constraint checking (ap2.sdk.constraints)."""
 
 import time
+from datetime import UTC, datetime
 
 import pytest
 
@@ -534,8 +535,8 @@ def test_payment_execution_date_after_window():
     assert any('after allowed window' in v for v in violations)
 
 
-def test_payment_execution_date_missing_passes():
-    """Missing execution date (immediate) passes."""
+def test_payment_execution_date_missing_uses_current_time_within_window():
+    """Immediate execution within the authorized window passes."""
     violations = check_payment_constraints(
         _open_payment(
             constraints=[
@@ -543,6 +544,52 @@ def test_payment_execution_date_missing_passes():
             ]
         ),
         _closed_payment(execution_date=None),
+        current_time=datetime(2025, 6, 1, tzinfo=UTC),
+    )
+    assert violations == []
+
+
+def test_payment_execution_date_missing_before_window():
+    """Immediate execution before the authorized window is a violation."""
+    violations = check_payment_constraints(
+        _open_payment(
+            constraints=[
+                ExecutionDate(not_before='2025-01-01', not_after='2025-12-31'),
+            ]
+        ),
+        _closed_payment(execution_date=None),
+        current_time=datetime(2024, 12, 31, tzinfo=UTC),
+    )
+    assert any('before allowed window' in v for v in violations)
+
+
+def test_payment_execution_date_missing_after_window():
+    """Immediate execution after the authorized window is a violation."""
+    violations = check_payment_constraints(
+        _open_payment(
+            constraints=[
+                ExecutionDate(not_before='2025-01-01', not_after='2025-12-31'),
+            ]
+        ),
+        _closed_payment(execution_date=None),
+        current_time=datetime(2026, 1, 1, tzinfo=UTC),
+    )
+    assert any('after allowed window' in v for v in violations)
+
+
+def test_payment_execution_date_missing_at_exact_boundary():
+    """Immediate execution at an ISO 8601 boundary passes."""
+    violations = check_payment_constraints(
+        _open_payment(
+            constraints=[
+                ExecutionDate(
+                    not_before='2025-06-01T00:00:00Z',
+                    not_after='2025-06-01T00:00:00Z',
+                ),
+            ]
+        ),
+        _closed_payment(execution_date=None),
+        current_time=datetime(2025, 6, 1, tzinfo=UTC),
     )
     assert violations == []