diff --git a/cal_tools/cal_tools/tools.py b/cal_tools/cal_tools/tools.py
index d88ca857ef35583728c7881ee8dbbd2ee877f82a..5f190d55780946bbf880842c10ba407c41009f40 100644
--- a/cal_tools/cal_tools/tools.py
+++ b/cal_tools/cal_tools/tools.py
@@ -8,7 +8,7 @@ from os.path import isfile
 from pathlib import Path
 from queue import Queue
 from time import sleep
-from typing import Optional, Union
+from typing import Union
 from urllib.parse import urljoin
 
 import dateutil.parser
@@ -237,8 +237,8 @@ def get_dir_creation_date(directory: Union[str, Path], run: int,
     """
     Return run start time from MyDC.
     If not available from MyMDC, retrieve the data from the dataset's metadata
-    in [directory]/[run] or, if the dataset is older than 2020, from the
-    directory's creation time.
+    in [directory]/[run] or, if the dataset is older than 2020, from the oldest
+    file's modified time.
 
     If the data is not available from either source, this function will raise a
     ValueError.
@@ -267,18 +267,18 @@ def get_dir_creation_date(directory: Union[str, Path], run: int,
     while ntries > 0:
         try:
             rfiles = list(directory.glob('*.h5'))
-            rfiles.sort(key=path.getmtime)
             # get creation time for oldest file,
             # as creation time between run files
-            # should be different only within few seconds
-            with h5py.File(rfiles[0], 'r') as fin:
+            # should differ by a few seconds only.
+            rfile = sorted(rfiles, key=path.getmtime)[0]
+            with h5py.File(rfile, 'r') as fin:
                 cdate = fin['METADATA/creationDate'][0].decode()
                 cdate = datetime.datetime.strptime(cdate, "%Y%m%dT%H%M%SZ")
             return cdate
         except (IndexError, IOError, ValueError):
             ntries -= 1
         except KeyError:  # The files are here, but it's an older dataset
-            return datetime.datetime.fromtimestamp(directory.stat().st_ctime)
+            return datetime.datetime.fromtimestamp(rfile.stat().st_mtime)
 
     msg = 'Could not get the creation time from the directory'
     raise ValueError(msg, directory)
diff --git a/tests/test_cal_tools.py b/tests/test_cal_tools.py
index 208066682fdbb409c97df7106ce567e68a8b81a7..24d61719ce141e78e517e32cc02816e02228f40f 100644
--- a/tests/test_cal_tools.py
+++ b/tests/test_cal_tools.py
@@ -29,4 +29,4 @@ def test_dir_creation_date():
     # The following data predates the addition of creation_time in metadata
     date = get_dir_creation_date(folder, 9999)
     assert isinstance(date, datetime)
-    assert str(date) == '2021-01-25 20:30:52.818820'
+    assert str(date) == '2019-12-16 08:52:25.196603'