diff --git a/src/subscript/fmuobs/fmuobs.py b/src/subscript/fmuobs/fmuobs.py
index 7f4518277..5954d7e38 100644
--- a/src/subscript/fmuobs/fmuobs.py
+++ b/src/subscript/fmuobs/fmuobs.py
@@ -335,10 +335,10 @@ def fmuobs(
                     starttime=starttime,
                 )
         else:
-            dframe = ertobs2df(input_str, cwd=includedir)
+            dframe = ertobs2df(input_str, cwd=includedir, starttime=starttime)
 
     if starttime:
-        dframe = compute_date_from_days(dframe)
+        dframe = compute_date_from_days(dframe, starttime=starttime)
 
     if not validate_internal_dframe(dframe):
         logger.error("Observation dataframe is invalid!")
diff --git a/tests/test_fmuobs.py b/tests/test_fmuobs.py
index ad26e75ef..4fb9d26a1 100644
--- a/tests/test_fmuobs.py
+++ b/tests/test_fmuobs.py
@@ -247,6 +247,55 @@ def test_integration():
     assert subprocess.check_output(["fmuobs", "-h"])
 
 
+@pytest.mark.integration
+def test_startdate_argument(tmp_path, mocker, monkeypatch):
+    """Test that the startdate argument is handled correctly"""
+    monkeypatch.chdir(tmp_path)
+    # Test that summary observations without a date are not valid without
+    # startdate argument
+    mocker.patch(
+        "sys.argv",
+        [
+            "fmuobs",
+            "--yml",
+            "output.yml",
+            str(
+                TESTDATA_DIR / "summary_obs_without_date.obs",
+            ),
+        ],
+    )
+    with pytest.raises(
+        ValueError, match=r"Can't have summary observation without a date"
+    ):
+        main()
+    # Test that summary observations without a date are valid with startdate argument
+    start_date = "2020-01-01"
+    mocker.patch(
+        "sys.argv",
+        [
+            "fmuobs",
+            "--startdate",
+            start_date,
+            "--yml",
+            "output.yml",
+            str(
+                TESTDATA_DIR / "summary_obs_without_date.obs",
+            ),
+        ],
+    )
+    main()
+    assert Path("output.yml").exists()
+    # Test that the dates in the output are correctly calculated
+    # from the startdate and the days in the input
+    with open(Path("output.yml"), encoding="utf8") as stream:
+        config = yaml.safe_load(stream)
+    for smry in config["smry"]:
+        for obs in smry["observations"]:
+            assert pd.to_datetime(obs["date"]) == pd.to_datetime(
+                start_date
+            ) + pd.to_timedelta(obs["days"], unit="D")
+
+
 @pytest.mark.integration
 @pytest.mark.parametrize("verbose", ["", "--verbose", "--debug"])
 def test_commandline(tmp_path, verbose, mocker, caplog, monkeypatch):
diff --git a/tests/testdata_fmuobs/summary_obs_without_date.obs b/tests/testdata_fmuobs/summary_obs_without_date.obs
new file mode 100644
index 000000000..6e14a1dd1
--- /dev/null
+++ b/tests/testdata_fmuobs/summary_obs_without_date.obs
@@ -0,0 +1,28 @@
+ -- These observations are modified from ert-doc.obs
+ -- and stripped down to demonstrate summary observations without dates
+ 
+ SUMMARY_OBSERVATION SEP_TEST_2005
+ {
+    VALUE = 100.0;
+    ERROR =     5;
+    DAYS  = 135;
+    KEY   = GOPR:BRENT;
+ };
+ 
+ SUMMARY_OBSERVATION SEP_TEST_2006
+ {
+    VALUE = 100.0;
+    ERROR =     5;
+    DAYS  = 500;
+    KEY   = GOPR:BRENT;
+ };
+ 
+ SUMMARY_OBSERVATION SEP_TEST_2008
+ {
+    VALUE    = 213;
+    ERROR    =  10;
+    DAYS     = 911;
+    KEY      = GOPR:NESS;
+ };
+
+ 
\ No newline at end of file