diff --git a/src/borg/constants.py b/src/borg/constants.py
index 6c13a9c98..36d6ee45b 100644
--- a/src/borg/constants.py
+++ b/src/borg/constants.py
@@ -66,6 +66,12 @@
 EXIT_WARNING = 1  # reached normal end of operation, but there were issues
 EXIT_ERROR = 2  # terminated abruptly, did not reach end of operation
 
+# never use datetime.isoformat(), it is evil. always use one of these:
+# datetime.strftime(ISO_FORMAT)  # output always includes .microseconds
+# datetime.strftime(ISO_FORMAT_NO_USECS)  # output never includes microseconds
+ISO_FORMAT_NO_USECS = '%Y-%m-%dT%H:%M:%S'
+ISO_FORMAT = ISO_FORMAT_NO_USECS + '.%f'
+
 DASHES = '-' * 78
 
 PBKDF2_ITERATIONS = 100000
diff --git a/src/borg/testsuite/archiver.py b/src/borg/testsuite/archiver.py
index 1bf3ed737..7a5b616a4 100644
--- a/src/borg/testsuite/archiver.py
+++ b/src/borg/testsuite/archiver.py
@@ -61,8 +61,6 @@
 
 src_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), '..'))
 
-ISO_FORMAT = '%Y-%m-%dT%H:%M:%S.%f'
-
 
 def exec_cmd(*args, archiver=None, fork=False, exe=None, input=b'', binary_output=False, **kw):
     if fork: