diff --git a/python_modules/dagster/dagster/core/test_utils.py b/python_modules/dagster/dagster/core/test_utils.py
index <HASH>..<HASH> 100644
--- a/python_modules/dagster/dagster/core/test_utils.py
+++ b/python_modules/dagster/dagster/core/test_utils.py
@@ -262,7 +262,8 @@ def new_cwd(path):
         os.chdir(old)
 
 
-def today_at_midnight(timezone_name=None):
+def today_at_midnight(timezone_name="UTC"):
+    check.str_param(timezone_name, "timezone_name")
     now = pendulum.now(timezone_name)
     return create_pendulum_time(now.year, now.month, now.day, tz=now.timezone.name)
Change today_at_midnight default timezone to UTC

Summary: This made sense in the old dark days when schedules defaulted to system timezone, but now we are enlightened and use UTC as the default. Make the tests that rely on this function also use UTC.

Test Plan: BK

Reviewers: prha, alangenfeld, johann, sashank

Reviewed By: johann

Differential Revision: <URL>