Fix missing round when converting expr to 'dt'.

qiskit/transpiler/passes/scheduling/time_unit_conversion.py

@@ -95,15 +95,24 @@ def run(self, dag: DAGCircuit):
 
                 visitor = _EvalDurationImpl(inst_durations.dt)
                 duration = node.op.duration.accept(visitor)
+                if visitor.in_cycles():
+                    has_dt = True
+                    # We need to round in case the expression evaluated to a non-integral 'dt'.
+                    rounded_duration = round(duration)
+                    rounding_error = abs(duration - rounded_duration)
+                    if rounding_error > 1e-15:
+                        warnings.warn(
+                            f"Duration is rounded to {rounded_duration:d} [dt] from {duration:f}",
+                            UserWarning,
+                        )
+                    duration = rounded_duration
+                else:
+                    has_si = True
                 if duration < 0:
                     raise TranspilerError(
                         f"Expression '{node.op.duration}' resolves to a negative duration!"
                     )
                 expression_durations[node._node_id] = duration
-                if visitor.in_cycles():
-                    has_dt = True
-                else:
-                    has_si = True
             else:
                 if node.op.unit == "dt":
                     has_dt = True

test/python/compiler/test_transpiler.py

@@ -1530,7 +1530,7 @@ def test_delay_converts_to_dt(self):
         out = transpile(qc, dt=1e-9, seed_transpiler=42)
         self.assertEqual(out.data[0].operation.unit, "dt")
 
-    def test_delay_converts_to_dt_expr(self):
+    def test_delay_converts_expr_to_dt(self):
         """Test that a delay instruction with a duration expression of type Duration
         is converted to units of dt given a backend."""
         qc = QuantumCircuit(2)
@@ -1545,6 +1545,22 @@ def test_delay_converts_to_dt_expr(self):
         out = transpile(qc, dt=1e-9, seed_transpiler=42)
         self.assertEqual(out.data[0].operation.unit, "dt")
 
+    def test_delay_converts_expr_to_dt_with_rounding(self):
+        """Test that converting to 'dt' from wall-time correctly rounds to nearest
+        integer."""
+        qc = QuantumCircuit(2)
+        qc.delay(expr.lift(Duration.ns(1234560)), [0])
+
+        backend = GenericBackendV2(num_qubits=4)
+        backend.target.dt = 5e-7
+
+        with self.assertWarnsRegex(UserWarning, "Duration is rounded"):
+            out = transpile(qc, backend, seed_transpiler=42)
+
+        self.assertEqual(out.data[0].operation.unit, "dt")
+        self.assertEqual(type(out.data[0].duration), int)
+        self.assertEqual(out.data[0].duration, round(float(1234560) / 1e9 / 5e-7))
+
     def test_delay_expr_evaluation_dt(self):
         """Test that a delay instruction with a complex duration expression
         of type Duration is evaluated to 'dt' properly."""