--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/lib-stat.py	Fri Jan 04 22:09:06 2019 +0100
@@ -0,0 +1,56 @@
+from __future__ import absolute_import, print_function
+
+
+import sys
+import os
+import datetime
+
+
+class FloatTimesInStat(object):
+    """Context manager to ensure that stat returns float values.
+
+    For 3.7 <= Mercurial < 4.6: Mercurial calls :func:`os.stat_float_times`
+    for `stat` and friends to return :class:`int` values.
+
+    Temporarily fix this.
+
+    """
+
+    __slots__ = ("_do_reset",)
+
+    def __init__(self):
+        self._do_reset = False
+
+    def __enter__(self):
+        if not os.stat_float_times():
+            os.stat_float_times(True)
+            self._do_reset = True
+        return self
+
+    def __exit__(self, *args):
+        if self._do_reset:
+            os.stat_float_times(False)
+
+
+def to_isoformat(t):
+    """Return the POSIX timestamp `t` formatted in full ISO format.
+
+    `t` is expected to be in the UTC timezone.
+
+    """
+    dt = datetime.datetime.utcfromtimestamp(t)
+    if dt.utcoffset() is None:
+        return dt.isoformat() + "Z"
+    else:
+        return dt.isoformat()
+
+
+def main():
+    with FloatTimesInStat():
+        for f in sys.argv[1:]:
+            st = os.lstat(f)
+            print(to_isoformat(st.st_mtime))
+
+
+if __name__ == "__main__":
+    main()

--- a/tests/test-timestamps.t	Fri Jan 04 21:26:23 2019 +0100
+++ b/tests/test-timestamps.t	Fri Jan 04 22:09:06 2019 +0100
@@ -3,6 +3,18 @@
 
   $ . $TESTDIR/lib-test-timestamps.sh
 
+Determine a python executable for usage in the shell
+
+  >>> from __future__ import print_function, absolute_import
+  >>> import io, os, sys
+  >>> if sys.version_info[0] < 3:
+  ...     u = unicode
+  ... else:
+  ...     u = str
+  >>> with io.open(os.path.join(os.environ["TESTTMP"], "tspy"), "wt", encoding="us-ascii") as tspy:
+  ...     print(u(sys.executable), file=tspy, end=u(''))
+  $ TSPY=`cat $TESTTMP/tspy`; export TSPY
+
 
 2. Create a test repository with output check
 =============================================
@@ -51,11 +63,65 @@
   8
 
 
+2. Test with updates and reverts
+================================
+
+  $ prepare_repo "ts2"
+  $ cd ts2
+
+Updating ...
+
+  $ hg id -n
+  8
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test1.txt
+  2017-12-01T02:03:04Z
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test2.txt
+  2017-12-01T08:09:10Z
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test3.txt
+  2017-12-01T05:06:07Z
+  $ hg update --quiet -r7
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test1.txt
+  2017-12-01T02:03:04Z
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test2.txt
+  2017-12-01T07:08:09Z
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test4-2.txt
+  2017-12-01T06:07:08Z
+  $ hg update --quiet -r4
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test1.txt
+  2017-12-01T02:03:04Z
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test2.txt
+  2017-12-01T04:05:06Z
+
+Reverting ...
+
+  $ hg update --quiet -rtip
+  $ hg id -n
+  8
+  $ echo CHANGED... >files/test1.txt
+  $ hg st
+  M files/test1.txt
+  $ hg revert -C files/test1.txt
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test1.txt
+  2017-12-01T02:03:04Z
+  $ hg st
+  $ echo "CHANGED 2 ..." >files/test1.txt
+  $ echo "CHANGED 2 ..." >>files/test2.txt
+  $ hg st
+  M files/test1.txt
+  M files/test2.txt
+  $ hg revert --all -C --quiet
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test1.txt
+  2017-12-01T02:03:04Z
+  $ "$TSPY" "$TESTDIR/lib-stat.py" files/test2.txt
+  2017-12-01T08:09:10Z
+  $ cd ..
+
+
 3. Test a failing merge
 =======================
 
-  $ prepare_repo "ts2"
-  $ cd ts2
+  $ prepare_repo "ts3"
+  $ cd ts3
   $ hg merge -r7 --tool=:merge3
   merging .hgtimestamps.db
   merging files/test2.txt