diff --git a/ptulsconv/pdf/__init__.py b/ptulsconv/pdf/__init__.py
index cb3aea3..3cd319c 100644
--- a/ptulsconv/pdf/__init__.py
+++ b/ptulsconv/pdf/__init__.py
@@ -94,6 +94,9 @@ def make_doc_template(page_size, filename, document_title,
 
 
 def time_format(mins, zero_str="-"):
+    """
+    Formats a duration `mins` into a string
+    """
     if mins == 0. and zero_str is not None:
         return zero_str
     elif mins < 60.: