diff --git a/requirements.txt b/requirements.txt
index d5ffdb9..c199fc3 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 # Add here external Python modules dependencies, one per line. Module names
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
+iso8601
 methodtools