diff --git a/NEWS b/NEWS
index 3e0ff16..b4c9eaa 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,10 @@ Improvements
   This was a side effect of the fix to bug #941958, where we fixed a cosmetic
   error. (Robert Collins, #1430534)
 
+* During reporting in ``TextTestResult`` now always uses ``ceil`` rather than
+  depending on the undefined rounding behaviour in string formatting.
+  (Robert Collins)
+
 1.7.0
 ~~~~~
 
diff --git a/testtools/testresult/real.py b/testtools/testresult/real.py
index 3b9ef4e..44e3468 100644
--- a/testtools/testresult/real.py
+++ b/testtools/testresult/real.py
@@ -23,6 +23,7 @@ __all__ = [
     ]
 
 import datetime
+import math
 from operator import methodcaller
 import sys
 import unittest
@@ -888,9 +889,14 @@ class TextTestResult(TestResult):
         self.sep1 = '=' * 70 + '\n'
         self.sep2 = '-' * 70 + '\n'
 
-    def _delta_to_float(self, a_timedelta):
-        return (a_timedelta.days * 86400.0 + a_timedelta.seconds +
-            a_timedelta.microseconds / 1000000.0)
+    def _delta_to_float(self, a_timedelta, precision):
+        # This calls ceiling to ensure that the most pessimistic view of time
+        # taken is shown (rather than leaving it to the Python %f operator
+        # to decide whether to round/floor/ceiling. This was added when we
+        # had pyp3 test failures that suggest a floor was happening.
+        shift = 10 ** precision
+        return math.ceil((a_timedelta.days * 86400.0 + a_timedelta.seconds +
+            a_timedelta.microseconds / 1000000.0) * shift) / shift
 
     def _show_list(self, label, error_list):
         for test, output in error_list:
@@ -918,7 +924,7 @@ class TextTestResult(TestResult):
                     self.sep1, test.id(), self.sep2))
         self.stream.write("\nRan %d test%s in %.3fs\n" %
             (self.testsRun, plural,
-             self._delta_to_float(stop - self.__start)))
+             self._delta_to_float(stop - self.__start, 3)))
         if self.wasSuccessful():
             self.stream.write("OK\n")
         else: