diff --git a/dfm_tools/download.py b/dfm_tools/download.py
index 7440a3a5a..5e44a14a6 100644
--- a/dfm_tools/download.py
+++ b/dfm_tools/download.py
@@ -297,3 +297,16 @@ def download_OPeNDAP(dataset_url,
         print(f'xarray writing netcdf file: {name_output}')
         data_xr_var_seltime.to_netcdf(os.path.join(dir_output,name_output)) #TODO: add chunks={'time':1} or only possible with opening?
         data_xr_var_seltime.close()
+
+
+def round_timestamp_to_outer_noon(date_min, date_max):
+    """
+    Since the CMEMS dataset only contains noon-values, we often need to round to the previous or next noon timestep to download enough data.
+    
+    """
+    
+    td_12h = pd.Timedelta(hours=12)
+    date_min = (pd.Timestamp(date_min) + td_12h).floor('1d') - td_12h
+    date_max = (pd.Timestamp(date_max) - td_12h).ceil('1d') + td_12h
+    return date_min, date_max
+
diff --git a/tests/test_download.py b/tests/test_download.py
new file mode 100644
index 000000000..fc38a3615
--- /dev/null
+++ b/tests/test_download.py
@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+"""
+Created on Thu Jun 22 09:41:49 2023
+
+@author: veenstra
+"""
+
+import pytest
+import pandas as pd
+import numpy as np
+from dfm_tools.download import round_timestamp_to_outer_noon
+
+
+@pytest.mark.unittest
+def test_round_timestamp_to_outer_noon():
+    date_min_mod_list = pd.date_range('2020-01-01','2020-01-02',freq='30min')
+    date_max_mod_list = pd.date_range('2020-01-03','2020-01-04',freq='30min')
+    
+    for date_min_mod, date_max_mod in zip(date_min_mod_list, date_max_mod_list):
+        
+        date_min, date_max = round_timestamp_to_outer_noon(date_min_mod, date_max_mod)
+        
+        assert date_min.hour == 12
+        assert date_min.minute == 0
+        assert date_max.hour == 12
+        assert date_max.minute == 0
+        assert date_min <= pd.Timestamp(date_min_mod)
+        assert date_max >= pd.Timestamp(date_max_mod)
+        assert np.abs((date_min - pd.Timestamp(date_min_mod)).total_seconds()) < 24*3600
+        assert np.abs((date_max - pd.Timestamp(date_max_mod)).total_seconds()) < 24*3600
\ No newline at end of file