diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index 06c29229..aa161dbc 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -303,7 +303,7 @@ 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)
+                window_interval = models.PeriodicInterval(self.windows_frequency, self.windows_duration, min(self.infected_start, self.exposed_start))
             else:
                 window_interval = always_on
 
diff --git a/cara/models.py b/cara/models.py
index 272b6b47..012e453e 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -123,11 +123,14 @@ class PeriodicInterval(Interval):
     #: occurring, a value of 0 signifies that the event never happens.
     duration: float
 
+    #: Time at which the first person (infected or exposed) arrives at the enclosed space.
+    start: float = 0.0
+
     def boundaries(self) -> BoundarySequence_t:
         if self.period == 0 or self.duration == 0:
             return tuple()
         result = []
-        for i in np.arange(0, 24, self.period / 60):
+        for i in np.arange(self.start, 24, self.period / 60):
             # NOTE: It is important that the time type is float, not np.float, in
             # order to allow hashability (for caching).
             result.append((float(i), float(i+self.duration/60)))
diff --git a/cara/tests/apps/calculator/test_model_generator.py b/cara/tests/apps/calculator/test_model_generator.py
index 6ee821c0..dd635946 100644
--- a/cara/tests/apps/calculator/test_model_generator.py
+++ b/cara/tests/apps/calculator/test_model_generator.py
@@ -48,7 +48,7 @@ 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),
+        active=models.PeriodicInterval(period=120, duration=10, start=minutes_since_midnight(9 * 60)),
         inside_temp=models.PiecewiseConstant((0, 24), (293,)),
         outside_temp=baseline_window.outside_temp,
         window_height=1.6, opening_length=0.6,
@@ -80,7 +80,7 @@ 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),
+        active=models.PeriodicInterval(period=120, duration=10, start=minutes_since_midnight(9 * 60)),
         inside_temp=models.PiecewiseConstant((0, 24), (293,)),
         outside_temp=baseline_window.outside_temp,
         window_height=1.6, window_width=1., opening_length=0.6,
@@ -141,7 +141,7 @@ 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),
+        active=models.PeriodicInterval(period=120, duration=10, start=minutes_since_midnight(9 * 60)),
         inside_temp=models.PiecewiseConstant((0, 24), (293,)),
         outside_temp=baseline_window.outside_temp,
         window_height=1.6, opening_length=0.6,