diff --git a/podaac/subsetter/gpm_cleanup.py b/podaac/subsetter/gpm_cleanup.py
index 456c08d7..c7b85404 100644
--- a/podaac/subsetter/gpm_cleanup.py
+++ b/podaac/subsetter/gpm_cleanup.py
@@ -8,8 +8,14 @@
 
 dim_dict = {}
 
-def create_new_time_var(time_group, nc_dataset):
+def compute_new_time_data(time_group, nc_dataset):
+    """
+    create a time variable, timeMidScan, that is present in other
+    GPM collections but not the ENV collections.
+    """
+    # set the time unit for GPM
     time_unit_out = "seconds since 1980-01-06 00:00:00"
+    # conver to a float, seconds variable
     new_time_list = [date2num(datetime.datetime(nc_dataset[time_group+'__Year'][:][i], nc_dataset[time_group+'__Month'][:][i], nc_dataset[time_group+'__DayOfMonth'][:][i],
                                             hour=nc_dataset[time_group+'__Hour'][:][i], minute=nc_dataset[time_group+'__Minute'][:][i],
                                             second=nc_dataset[time_group+'__Second'][:][i], microsecond=nc_dataset[time_group+'__Second'][:][i]*1000), time_unit_out) 
@@ -74,14 +80,19 @@ def change_var_dims(nc_dataset, variables=None):
                 new_mapped_var[var_name][:] = var[:]
 
     if not any("timeMidScan" in var for var in var_list):
+        # if there isn't any timeMidScan variables, create one
         scan_time_groups = ["__".join(i.split('__')[:-1]) for i in var_list if 'ScanTime' in i]
         for time_group in list(set(scan_time_groups)):
-            time_data, time_unit = create_new_time_var(time_group, nc_dataset)
+            # get the seconds since Jan 6, 1980
+            time_data, time_unit = compute_new_time_data(time_group, nc_dataset)
+            # make a new variable for each ScanTime group
             new_time_var_name = time_group+'__timeMidScan'
+            # copy dimensions from the Year variable
             var_dims = nc_dataset.variables[time_group+'__Year'].dimensions
             comp_args = {"zlib": True, "complevel": 1}
             nc_dataset.createVariable(new_time_var_name, 'f8', var_dims, **comp_args)
             nc_dataset.variables[new_time_var_name].setncattr('unit', time_unit)
+            # copy the data in
             nc_dataset.variables[new_time_var_name][:] = time_data
 
     return nc_dataset
diff --git a/tests/test_subset.py b/tests/test_subset.py
index 17f51102..e1485e0b 100644
--- a/tests/test_subset.py
+++ b/tests/test_subset.py
@@ -2309,3 +2309,22 @@ def test_gpm_dimension_map(data_dir, subset_output_dir, request):
         
         for dim in dims:
             assert 'phony' not in dim
+
+def test_gpm_compute_new_var_data(data_dir, subset_output_dir, request):
+    """Test GPM files that have scantime variable to compute the time for seconds
+    since 1980-01-06"""
+    
+    gpm_dir = join(data_dir, 'GPM')
+    gpm_file = 'GPM_test_file_3.HDF5'
+    bbox = np.array(((-180, 180), (-90, 90)))
+    shutil.copyfile(
+        os.path.join(gpm_dir, gpm_file),
+        os.path.join(subset_output_dir, gpm_file)
+    )
+
+    nc_dataset, has_groups, file_extension = subset.open_as_nc_dataset(join(subset_output_dir, gpm_file))
+
+
+    time_data, time_unit = gc.compute_new_time_data("__FS__ScanTime", nc_dataset)
+
+    assert int(time_data[0]) == 1325120552