diff --git a/augur/dates/__init__.py b/augur/dates/__init__.py
index 046638826..57b820a5e 100644
--- a/augur/dates/__init__.py
+++ b/augur/dates/__init__.py
@@ -144,4 +144,5 @@ def get_numerical_dates(metadata:pd.DataFrame, name_col = None, date_col='date',
     return dict(zip(strains, dates))
 
 def get_iso_year_week(year, month, day):
-    return datetime.date(year, month, day).isocalendar()[:2]
+    year, week = datetime.date(year, month, day).isocalendar()[:2]
+    return f"{year}-{week}"