diff --git a/src/query/validation.c b/src/query/validation.c
index 82942523..7b5baf22 100644
--- a/src/query/validation.c
+++ b/src/query/validation.c
@@ -427,7 +427,9 @@ static Var *get_bucket_expression_column_ref(Node *bucket_expression)
     return (Var *)bucket_expression;
   /* If the bucket expression is not a direct column reference, it means it is a simple function call. */
   FuncExpr *func_expr = castNode(FuncExpr, bucket_expression);
-  return castNode(Var, unwrap_cast(linitial(func_expr->args)));
+
+  int primary_arg = primary_arg_index(func_expr->funcid);
+  return castNode(Var, unwrap_cast(list_nth(func_expr->args, primary_arg)));
 }
 
 static void verify_column_usage_in_filter(AccessLevel access_level, Node *bucket_expression, List *range_tables)
diff --git a/test/expected/datetime.out b/test/expected/datetime.out
index c03829a5..99bb8d0f 100644
--- a/test/expected/datetime.out
+++ b/test/expected/datetime.out
@@ -78,3 +78,10 @@ SELECT tz, count(*) FROM test_datetime GROUP BY 1;
  2012-05-14 07:00:00+00 |    11
 (1 row)
 
+-- Datetime filtering
+SELECT count(*) FROM test_datetime WHERE date_trunc('year', ts) = '2012-01-01'::timestamp;
+ count 
+-------
+    11
+(1 row)
+
diff --git a/test/sql/datetime.sql b/test/sql/datetime.sql
index ff714cf7..01482b86 100755
--- a/test/sql/datetime.sql
+++ b/test/sql/datetime.sql
@@ -53,3 +53,6 @@ SET pg_diffix.session_access_level = 'direct';
 UPDATE test_datetime SET tz = '2012-05-14T08:00+01:00' WHERE true;
 SET pg_diffix.session_access_level = 'anonymized_trusted';
 SELECT tz, count(*) FROM test_datetime GROUP BY 1;
+
+-- Datetime filtering
+SELECT count(*) FROM test_datetime WHERE date_trunc('year', ts) = '2012-01-01'::timestamp;