diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index c4593b49..196487bc 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -324,7 +324,7 @@ class FormData:
         # Initializes a ventilation instance as a window if 'natural_ventilation' is selected, or as a HEPA-filter otherwise
         if self.ventilation_type == 'natural_ventilation':
             if self.window_opening_regime == 'windows_open_periodically':
-                window_interval = models.PeriodicInterval(self.windows_frequency, self.windows_duration, min(self.infected_start, self.exposed_start))
+                window_interval = models.PeriodicInterval(self.windows_frequency, self.windows_duration, min(self.infected_start, self.exposed_start)/60)
             else:
                 window_interval = always_on
 
diff --git a/cara/tests/apps/calculator/test_model_generator.py b/cara/tests/apps/calculator/test_model_generator.py
index 5cc9e409..3bfb08f5 100644
--- a/cara/tests/apps/calculator/test_model_generator.py
+++ b/cara/tests/apps/calculator/test_model_generator.py
@@ -59,7 +59,7 @@ def test_ventilation_slidingwindow(baseline_form: model_generator.FormData):
     assert isinstance(baseline_window, models.SlidingWindow)
 
     window = models.SlidingWindow(
-        active=models.PeriodicInterval(period=120, duration=10, start=minutes_since_midnight(9 * 60)),
+        active=models.PeriodicInterval(period=120, duration=10, start=9),
         inside_temp=models.PiecewiseConstant((0, 24), (293,)),
         outside_temp=baseline_window.outside_temp,
         window_height=1.6, opening_length=0.6,
@@ -91,7 +91,7 @@ def test_ventilation_hingedwindow(baseline_form: model_generator.FormData):
     assert isinstance(baseline_window, models.HingedWindow)
 
     window = models.HingedWindow(
-        active=models.PeriodicInterval(period=120, duration=10, start=minutes_since_midnight(9 * 60)),
+        active=models.PeriodicInterval(period=120, duration=10, start=9),
         inside_temp=models.PiecewiseConstant((0, 24), (293,)),
         outside_temp=baseline_window.outside_temp,
         window_height=1.6, window_width=1., opening_length=0.6,
@@ -152,7 +152,7 @@ def test_ventilation_window_hepa(baseline_form: model_generator.FormData):
 
     # Now build the equivalent ventilation instance directly, and compare.
     window = models.SlidingWindow(
-        active=models.PeriodicInterval(period=120, duration=10, start=minutes_since_midnight(9 * 60)),
+        active=models.PeriodicInterval(period=120, duration=10, start=9),
         inside_temp=models.PiecewiseConstant((0, 24), (293,)),
         outside_temp=baseline_window.outside_temp,
         window_height=1.6, opening_length=0.6,