diff --git a/qcelemental/physical_constants/ureg.py b/qcelemental/physical_constants/ureg.py
index 3e000e7a..b82af0da 100644
--- a/qcelemental/physical_constants/ureg.py
+++ b/qcelemental/physical_constants/ureg.py
@@ -43,6 +43,9 @@ def build_units_registry(context):
     ureg.define("wavenumber = 1 / centimeter")
     ureg.define("Angstrom = angstrom")
 
+    # Time
+    ureg.define(f"au_time = {phys_const['atomic unit of time']['value']} * {phys_const['atomic unit of time']['unit']}")
+
     # Masses
     ureg.define(
         "atomic_mass_unit = {} * kilogram = u = amu = dalton = Da".format(phys_const["atomic mass constant"]["value"])
diff --git a/qcelemental/tests/test_units.py b/qcelemental/tests/test_units.py
index 0b19334b..b1cabbfb 100644
--- a/qcelemental/tests/test_units.py
+++ b/qcelemental/tests/test_units.py
@@ -78,3 +78,14 @@ def test_quantities_smoke():
     Smoke test to ensure Quantities are correctly returned
     """
     assert 5 == qcelemental.constants.Quantity("5 kcal").magnitude
+
+
+def test_speed_of_light():
+    assert (
+        pytest.approx(
+            qcelemental.constants.speed_of_light_in_vacuum
+            * qcelemental.constants.conversion_factor("m/s", "bohr/au_time"),
+            1e-8,
+        )
+        == qcelemental.constants.c_au
+    )