diff --git a/resync/w3c_datetime.py b/resync/w3c_datetime.py
index 7896d88..bd4b68a 100644
--- a/resync/w3c_datetime.py
+++ b/resync/w3c_datetime.py
@@ -32,6 +32,8 @@ def datetime_to_str(dt='now',no_fractions=False):
         dt = time.time()
     if (no_fractions):
         dt = int(dt)
+    else:
+        dt += 0.0000001 # improve rounding to microseconds
     return datetime.utcfromtimestamp(dt).isoformat() + 'Z'
 
 def str_to_datetime(s, context='datetime'):
diff --git a/tests/test_w3c_datetime.py b/tests/test_w3c_datetime.py
index b8d1350..474970f 100644
--- a/tests/test_w3c_datetime.py
+++ b/tests/test_w3c_datetime.py
@@ -21,6 +21,16 @@ class TestW3cDatetime(unittest.TestCase):
         self.assertEqual( datetime_to_str(60*60*24*365), "1971-01-01T00:00:00Z" )
         #
         self.assertEqual( datetime_to_str(1234567890), "2009-02-13T23:31:30Z" )
+        # Rounding issues
+        self.assertEqual( datetime_to_str(0.199999),  "1970-01-01T00:00:00.199999Z" )
+        self.assertEqual( datetime_to_str(0.1999991), "1970-01-01T00:00:00.199999Z" )
+        self.assertEqual( datetime_to_str(0.1999992), "1970-01-01T00:00:00.199999Z" )
+        self.assertEqual( datetime_to_str(0.1999998), "1970-01-01T00:00:00.200000Z" )
+        self.assertEqual( datetime_to_str(0.1999999), "1970-01-01T00:00:00.200000Z" )
+        self.assertEqual( datetime_to_str(0.200000),  "1970-01-01T00:00:00.200000Z" )
+        self.assertEqual( datetime_to_str(0.2000001), "1970-01-01T00:00:00.200000Z" )
+        self.assertEqual( datetime_to_str(0.2000009), "1970-01-01T00:00:00.200001Z" )
+        self.assertEqual( datetime_to_str(0.200001),  "1970-01-01T00:00:00.200001Z" )
 
     def test2_str_to_datetime(self):
         """Reading..."""