diff --git a/hc/front/forms.py b/hc/front/forms.py
index daeca968..0783523b 100644
--- a/hc/front/forms.py
+++ b/hc/front/forms.py
@@ -1,5 +1,5 @@
 from django import forms
-from hc.front.validators import WebhookValidator
+from hc.front.validators import CronExpressionValidator, WebhookValidator
 from hc.api.models import CHECK_KINDS
 
 
@@ -21,7 +21,8 @@ class NameTagsForm(forms.Form):
 class TimeoutForm(forms.Form):
     kind = forms.ChoiceField(choices=CHECK_KINDS)
     timeout = forms.IntegerField(min_value=60, max_value=2592000)
-    schedule = forms.CharField(required=False, max_length=100)
+    schedule = forms.CharField(required=False, max_length=100,
+                               validators=[CronExpressionValidator()])
     tz = forms.CharField(required=False, max_length=36)
     grace = forms.IntegerField(min_value=60, max_value=2592000)
 
diff --git a/hc/front/tests/test_update_timeout.py b/hc/front/tests/test_update_timeout.py
index cea51751..c0e12b76 100644
--- a/hc/front/tests/test_update_timeout.py
+++ b/hc/front/tests/test_update_timeout.py
@@ -31,7 +31,7 @@ class UpdateTimeoutTestCase(BaseTestCase):
         url = "/checks/%s/timeout/" % self.check.code
         payload = {
             "kind": "cron",
-            "schedule": "* * * * *",
+            "schedule": "5 * * * *",
             "tz": "UTC",
             "timeout": 60,
             "grace": 60
@@ -43,7 +43,28 @@ class UpdateTimeoutTestCase(BaseTestCase):
 
         self.check.refresh_from_db()
         self.assertEqual(self.check.kind, "cron")
-        self.assertEqual(self.check.schedule, "* * * * *")
+        self.assertEqual(self.check.schedule, "5 * * * *")
+
+    def test_it_validates_cron_expression(self):
+        self.check.last_ping = None
+        self.check.save()
+
+        url = "/checks/%s/timeout/" % self.check.code
+        payload = {
+            "kind": "cron",
+            "schedule": "* invalid *",
+            "tz": "UTC",
+            "timeout": 60,
+            "grace": 60
+        }
+
+        self.client.login(username="alice@example.org", password="password")
+        r = self.client.post(url, data=payload)
+        self.assertRedirects(r, "/checks/")
+
+        # Check should still have its original data:
+        self.check.refresh_from_db()
+        self.assertEqual(self.check.kind, "simple")
 
     def test_team_access_works(self):
         url = "/checks/%s/timeout/" % self.check.code
diff --git a/hc/front/validators.py b/hc/front/validators.py
index bb9481fa..7cdefb9e 100644
--- a/hc/front/validators.py
+++ b/hc/front/validators.py
@@ -1,3 +1,4 @@
+from croniter import croniter
 from django.core.exceptions import ValidationError
 from six.moves.urllib_parse import urlparse
 
@@ -12,3 +13,13 @@ class WebhookValidator(object):
 
         if parsed.hostname in ("127.0.0.1", "localhost"):
             raise ValidationError(message=self.message)
+
+
+class CronExpressionValidator(object):
+    message = "Not a valid cron expression."
+
+    def __call__(self, value):
+        try:
+            croniter(value)
+        except:
+            raise ValidationError(message=self.message)
diff --git a/templates/front/my_checks.html b/templates/front/my_checks.html
index 61281e88..d24153c2 100644
--- a/templates/front/my_checks.html
+++ b/templates/front/my_checks.html
@@ -28,12 +28,10 @@
         {% endfor %}
     </div>
     {% endif %}
-
 </div>
+
 <div class="row">
     <div class="col-sm-12">
-
-
     {% if checks %}
         {% include "front/my_checks_mobile.html" %}
         {% include "front/my_checks_desktop.html" %}