diff --git a/src/uwtools/drivers/mpas.py b/src/uwtools/drivers/mpas.py
index 0fb9988cb..b752f7841 100644
--- a/src/uwtools/drivers/mpas.py
+++ b/src/uwtools/drivers/mpas.py
@@ -48,7 +48,7 @@ def namelist_file(self):
         yield asset(path, path.is_file)
         yield None
         duration = timedelta(hours=self._driver_config["length"])
-        str_duration = str(duration).replace(" days, ", "")
+        str_duration = str(duration).replace(" days, ", "_")
         try:
             namelist = self._driver_config["namelist"]
         except KeyError as e:
diff --git a/src/uwtools/tests/drivers/test_mpas.py b/src/uwtools/tests/drivers/test_mpas.py
index 257a352ba..2bafee2fc 100644
--- a/src/uwtools/tests/drivers/test_mpas.py
+++ b/src/uwtools/tests/drivers/test_mpas.py
@@ -135,7 +135,22 @@ def test_MPAS_namelist_file(caplog, driverobj):
     path = Path(refs(driverobj.namelist_file()))
     assert dst.is_file()
     assert logged(caplog, f"Wrote config to {path}")
-    assert isinstance(f90nml.read(dst), f90nml.Namelist)
+    nml = f90nml.read(dst)
+    assert isinstance(nml, f90nml.Namelist)
+
+
+def test_MPAS_namelist_file_long_duration(caplog, config, cycle):
+    log.setLevel(logging.DEBUG)
+    config["mpas"]["length"] = 120
+    driverobj = MPAS(config=config, cycle=cycle)
+    dst = driverobj._rundir / "namelist.atmosphere"
+    assert not dst.is_file()
+    path = Path(refs(driverobj.namelist_file()))
+    assert dst.is_file()
+    assert logged(caplog, f"Wrote config to {path}")
+    nml = f90nml.read(dst)
+    assert isinstance(nml, f90nml.Namelist)
+    assert nml["nhyd_model"]["config_run_duration"] == "5_0:00:00"
 
 
 def test_MPAS_namelist_file_fails_validation(caplog, driverobj):