diff --git a/nlmod/dims/grid.py b/nlmod/dims/grid.py
index de2fd6aa..4a258040 100644
--- a/nlmod/dims/grid.py
+++ b/nlmod/dims/grid.py
@@ -293,19 +293,25 @@ def modelgrid_to_vertex_ds(mg, ds, nodata=-1):
     return ds
 
 
-def modelgrid_to_ds(mg):
+def modelgrid_to_ds(mg=None, grbfile=None):
     """Create Dataset from flopy modelgrid object.
 
     Parameters
     ----------
     mg : flopy.discretization.Grid
         flopy modelgrid object
+    grbfile : str
+        path to a binary grid file
 
     Returns
     -------
     ds : xarray.Dataset
         Dataset containing grid information
     """
+    if mg is None and grbfile is not None:
+        mg = flopy.utils.MfGrdFile(grbfile).modelgrid
+    elif mg is None and grbfile is None:
+        raise ValueError("Either 'mg' or 'grbfile' should be specified!")
     if mg.grid_type == "structured":
         x, y = mg.xyedges
         from .base import _get_structured_grid_ds
diff --git a/nlmod/mfoutput/mfoutput.py b/nlmod/mfoutput/mfoutput.py
index 0e98c874..906f9d34 100644
--- a/nlmod/mfoutput/mfoutput.py
+++ b/nlmod/mfoutput/mfoutput.py
@@ -71,12 +71,18 @@ def _get_time_index(fobj, ds=None, gwf_or_gwt=None):
             time_units=gwf_or_gwt.modeltime.time_units,
         )
     elif ds is not None:
+        if "time" in ds:
+            dtype = "float" if ds.time.dtype.kind in ["i", "f"] else "datetime"
+        else:
+            dtype = "float"
         tindex = ds_time_idx(
             fobj.get_times(),
             start_datetime=(ds.time.attrs["start"] if "time" in ds else None),
             time_units=(ds.time.attrs["time_units"] if "time" in ds else None),
-            dtype="float" if ds.time.dtype.kind in ["i", "f"] else "datetime",
+            dtype=dtype,
         )
+    else:
+        raise ValueError("Provide either ds or gwf_or_gwt")
     return tindex