diff --git a/src/robofish/io/file.py b/src/robofish/io/file.py
index 14d2a4d1d3f2aec3253e84d6733c4ae3b6b6851a..5d5742ffd3f28d63e038178db0ab296927e6249e 100644
--- a/src/robofish/io/file.py
+++ b/src/robofish/io/file.py
@@ -234,10 +234,10 @@ class File(h5py.File):
             def format_calendar_time_point(p):
                 if isinstance(p, datetime.datetime):
                     assert p.tzinfo is not None, "Missing timezone for calendar point."
-                    return p.isoformat(timespec="milliseconds")
+                    return p.isoformat(timespec="microseconds")
                 elif isinstance(p, str):
                     assert p == datetime.datetime.fromisoformat(p).isoformat(
-                        timespec="milliseconds"
+                        timespec="microseconds"
                     )
                     return p
                 else:
@@ -249,8 +249,6 @@ class File(h5py.File):
                 format_calendar_time_point(p) for p in calendar_time_points
             ]
 
-            for c in calendar_time_points:
-                print(type(c))
             sampling.create_dataset(
                 "calendar_time_points",
                 data=calendar_time_points,
diff --git a/src/robofish/io/io.py b/src/robofish/io/io.py
index c8894ca38365340509b94e59b5ea85374357109d..4c12aded73ffab9675370b160dac54b832164331 100644
--- a/src/robofish/io/io.py
+++ b/src/robofish/io/io.py
@@ -16,7 +16,7 @@ def now_iso8061() -> str:
         str: The current time as iso8061 string.
     """
     return datetime.datetime.now(datetime.timezone.utc).isoformat(
-        timespec="milliseconds"
+        timespec="microseconds"
     )
 
 
diff --git a/tests/robofish/io/test_file.py b/tests/robofish/io/test_file.py
index fe6f1f9c5f8dbbcb88a3a9162aaa324da3c8df2e..c098d574d2d560f0d3e2ffb4eaa28e965a9e87e0 100644
--- a/tests/robofish/io/test_file.py
+++ b/tests/robofish/io/test_file.py
@@ -112,7 +112,7 @@ def test_multiple_entities():
     # create new sampling
     m_points = np.ones((timesteps))
     c_points = np.empty((timesteps), dtype="O")
-    c_points[:5] = "2020-12-02T10:21:58.100+00:00"
+    c_points[:5] = "2020-12-02T10:21:58.100000+00:00"
     c_points[5:] = robofish.io.now_iso8061()
 
     new_sampling = sf.create_sampling(
diff --git a/tests/robofish/io/test_io.py b/tests/robofish/io/test_io.py
index bb4cb69365d8bc8d325fb1bf7cd502fb0be536f3..3a7bb32390223adfbd4e7f2f1f9a3454c93bdfa4 100644
--- a/tests/robofish/io/test_io.py
+++ b/tests/robofish/io/test_io.py
@@ -6,10 +6,10 @@ import numpy as np
 
 
 def test_now_iso8061():
-    # Example time: 2021-01-05T14:33:40.401+00:00
+    # Example time: 2021-01-05T14:33:40.401000+00:00
     time = robofish.io.now_iso8061()
     assert type(time) == str
-    assert len(time) == 29
+    assert len(time) == 32
 
 
 def test_read_multiple_single():