diff --git a/mapping_functions.md b/mapping_functions.md
index 650e507..40da55e 100644
--- a/mapping_functions.md
+++ b/mapping_functions.md
@@ -200,7 +200,6 @@ Functions
     
     Args:
         data_values: a values dict with a date
-        date_format: string with order of day, month and year in date (e.g. "DMY", "YMD", etc.)
     
     Returns:
         A dictionary with calculated month_interval and optionally a day_interval depending on the specified
diff --git a/src/clinical_etl/mappings.py b/src/clinical_etl/mappings.py
index 907c32b..df35728 100644
--- a/src/clinical_etl/mappings.py
+++ b/src/clinical_etl/mappings.py
@@ -86,7 +86,7 @@ def earliest_date(data_values):
     }
 
 
-def date_interval(data_values, date_format=None):
+def date_interval(data_values):
     """Calculates a date interval from a given date relative to the reference date specified in the manifest.
 
     Args: