diff --git a/hatyan/timeseries.py b/hatyan/timeseries.py
index 5cbb84aa..67e9e5f7 100644
--- a/hatyan/timeseries.py
+++ b/hatyan/timeseries.py
@@ -235,14 +235,17 @@ def calc_HWLW12345to12(data_HWLW_12345):
     logger.info('starting HWLW 12345 to 12 correction')
     times_LWmin = []
     data_HW1 = data_HWLW_12345.loc[data_HWLW_12345['HWLWcode']==1]
-    #computing minimum waterlevels after each HW. Using hardcoded 12hour period instead of from one HW to next HW since then we can also assess last LW values
+    # computing minimum waterlevels after each HW. Using hardcoded 12hour period 
+    # instead of from one HW to next HW since then we can also assess last LW values
     for timeHW in data_HW1.index: #np.arange(0,len(data_HW1)-1):
         if timeHW==data_HWLW_12345.index[-1]: #if last HW is last time of input dataframe
             continue
-        #tide_afterHW = data_HWLW_12345.loc[data_HW1.index[iHW]:data_HW1.index[iHW+1]]
         tide_afterHW = data_HWLW_12345.loc[timeHW:timeHW+dt.timedelta(hours=12)]
-        tide_afterHW = tide_afterHW.iloc[1:] #remove first HW to avoid issues if LW is higher than HW due to surge
-        if len(tide_afterHW)==0: #this happens if there is no LW defined between two HWs, for instance after SCHEVNGN HW at '1948-04-30 19:50:00'
+        # remove first HW to avoid issues if LW is higher than HW due to surge
+        tide_afterHW = tide_afterHW.iloc[1:]
+        if len(tide_afterHW)==0:
+            # this happens if there is no LW defined between two HWs
+            # for instance after SCHEVNGN HW at '1948-04-30 19:50:00'
             continue
         time_minimum = tide_afterHW['values'].idxmin()
         times_LWmin.append(time_minimum)
diff --git a/tests/test_timeseries.py b/tests/test_timeseries.py
index d244ee9b..613608ed 100644
--- a/tests/test_timeseries.py
+++ b/tests/test_timeseries.py
@@ -492,7 +492,6 @@ def test_writenetcdf():
 
 @pytest.mark.unittest
 def test_writenetcdf_nosidx():
-    
     current_station = 'VLISSGN'
     
     file_pred = os.path.join(dir_testdata,f'{current_station}_pre.txt')
@@ -525,3 +524,87 @@ def test_writenetcdf_nosidx():
     data_nc.close()
     os.remove(file_nc)
 
+
+@pytest.mark.unittest
+def test_calc_HWLW12345to12():
+    file_ext = os.path.join(dir_testdata,'hoek_har.dia')
+    
+    df = hatyan.read_dia(file_ext, block_ids=0)
+    df_12 = hatyan.calc_HWLW12345to12(df)
+    
+    assert len(df) == 3977
+    assert len(df_12) == 2825
+
+
+@pytest.mark.unittest
+def test_calc_HWLW12345to12_include_last_lw():
+    file_ext = os.path.join(dir_testdata,'hoek_har.dia')
+    
+    df = hatyan.read_dia(file_ext, block_ids=0)
+    
+    # get timeseries that ends with HWLWcode=2
+    df_sel = df.iloc[:-1]
+    df_12 = hatyan.calc_HWLW12345to12(df_sel)
+    
+    assert len(df) == 3977
+    assert len(df_sel) == 3976
+    assert df_sel["HWLWcode"].iloc[0] == 1
+    assert df_sel["HWLWcode"].iloc[-1] == 2
+    assert len(df_12) == 2824
+    assert df_12["HWLWcode"].iloc[0] == 1
+    assert df_12["HWLWcode"].iloc[-1] == 2
+
+
+@pytest.mark.unittest
+def test_calc_HWLW12345to12_include_first_lw():
+    file_ext = os.path.join(dir_testdata,'hoek_har.dia')
+    
+    df = hatyan.read_dia(file_ext, block_ids=0)
+    
+    # get timeseries that starts with HWLWcode=2
+    df_sel = df.iloc[9:]
+    df_12 = hatyan.calc_HWLW12345to12(df_sel)
+    
+    assert len(df) == 3977
+    assert len(df_sel) == 3968
+    assert df_sel["HWLWcode"].iloc[0] == 2
+    assert df_sel["HWLWcode"].iloc[-1] == 1
+    assert len(df_12) == 2820
+    assert df_12["HWLWcode"].iloc[0] == 2
+    assert df_12["HWLWcode"].iloc[-1] == 1
+
+
+@pytest.mark.unittest
+def test_calc_HWLW12345to12_skip_missing_lw():
+    file_ext = os.path.join(dir_testdata,'hoek_har.dia')
+    
+    df = hatyan.read_dia(file_ext, block_ids=0)
+    
+    # construct boolean to drop the first low waters (345 combination)
+    # and the last low water (2)
+    bool_drop = df["HWLWcode"] != 0
+    bool_drop.iloc[1:4] = False
+    bool_drop.iloc[-2:-1] = False
+    
+    df_sel = df.loc[bool_drop]
+    df_12 = hatyan.calc_HWLW12345to12(df_sel)
+    
+    assert len(df) == 3977
+    assert len(df_sel) == 3973
+    assert df_sel["HWLWcode"].iloc[0:2].tolist() == [1,1]
+    assert df_sel["HWLWcode"].iloc[-2:].tolist() == [1,1]
+    assert len(df_12) == 2823
+    assert df_12["HWLWcode"].iloc[0:2].tolist() == [1,1]
+    assert df_12["HWLWcode"].iloc[-2:].tolist() == [1,1]
+
+
+@pytest.mark.unittest
+def test_calc_HWLW12345to12_already_12():
+    file_ext = os.path.join(dir_testdata,'VLISSGN_ext.txt')
+    
+    df = hatyan.read_dia(file_ext)
+    
+    df_12 = hatyan.calc_HWLW12345to12(df)
+    
+    assert len(df) == 1411
+    assert len(df_12) == 1411