diff --git a/src/oemof/solph/_helpers.py b/src/oemof/solph/_helpers.py
index 15066af0c..ceae7e269 100644
--- a/src/oemof/solph/_helpers.py
+++ b/src/oemof/solph/_helpers.py
@@ -104,7 +104,12 @@ def create_time_index(
         else:
             hoy = 8760
         number = round(hoy / interval)
-    if start is None:
+    if start is not None:
+        if year is not None:
+            raise ValueError(
+                "Arguments 'start' and 'year' are mutually exclusive."
+            )
+    else:
         start = f"1/1/{year}"
     try:
         time_index = pd.date_range(
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index c33380a17..f878896b1 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -9,8 +9,10 @@
 """
 
 import os
+import pytest
 
 from oemof.solph import helpers
+from oemof.solph import create_time_index
 
 
 def test_creation_of_extended_path():
@@ -18,3 +20,25 @@ def test_creation_of_extended_path():
     p = helpers.extend_basic_path("test_subfolder_X345qw34_tmp")
     assert os.path.isdir(p)
     os.rmdir(p)
+
+
+def test_create_time_index():
+    assert len(create_time_index(2014)) == 8761
+    assert len(create_time_index(2012)) == 8785  # leap year
+    assert len(create_time_index(2014, interval=0.5)) == 17521
+    assert len(create_time_index(2014, interval=0.5, number=10)) == 11
+    assert len(create_time_index(2014, number=10)) == 11
+    assert (
+        str(create_time_index(2014, interval=0.5, number=10)[-1])
+        == "2014-01-01 05:00:00"
+    )
+    assert (
+        str(create_time_index(2014, interval=2, number=10)[-1])
+        == "2014-01-01 20:00:00"
+    )
+    assert (
+        str(create_time_index(interval=0.5, number=10, start="2025-01-02")[-1])
+        == "2025-01-02 05:00:00"
+    )
+    with pytest.raises(ValueError, match="mutually exclusive"):
+        create_time_index(year=2015, start="2025-01-02")