diff --git a/flashcards/models.py b/flashcards/models.py
index 809fb61..ea9f165 100644
--- a/flashcards/models.py
+++ b/flashcards/models.py
@@ -483,17 +483,17 @@ class JulianDay(Func):
         super(JulianDay, self).__init__(expression, output_field=output, **extra)
 
 
-class EncapsulateIntervalDay(Func):
+class PostgresIntervalDay(Func):
     function = 'EXTRACT'
     template_postgres = "(((%(function)s(EPOCH FROM INTERVAL %(expressions)s) / 60) / 60) / 24)::integer"
     arg_joiner = ' - '
 
-    def __init__(self, expression, **extra):
+    def __init__(self, *expression, **extra):
         output = extra.pop('output_field', FloatField())
-        super(EncapsulateIntervalDay, self).__init__(expression, output_field=output, **extra)
+        super(PostgresIntervalDay, self).__init__(*expression, output_field=output, **extra)
 
 
 def interval_days(expression1, expression2):
     if IN_PRODUCTION:
-        return EncapsulateIntervalDay(expression1, expression2)
+        return PostgresIntervalDay(expression1, expression2)
     return JulianDay(expression1) - JulianDay(expression2)