diff --git a/hc/api/decorators.py b/hc/api/decorators.py
index 366a2636..ec4c7d8a 100644
--- a/hc/api/decorators.py
+++ b/hc/api/decorators.py
@@ -1,6 +1,5 @@
 import json
 import re
-import uuid
 from functools import wraps
 
 from django.contrib.auth.models import User
diff --git a/hc/api/models.py b/hc/api/models.py
index 8eddb2ae..5ce384f3 100644
--- a/hc/api/models.py
+++ b/hc/api/models.py
@@ -47,6 +47,11 @@ PO_PRIORITIES = {
 }
 
 
+def isostring(dt):
+    """Convert the datetime to ISO 8601 format with no microseconds. """
+    return dt.replace(microsecond=0).isoformat()
+
+
 class Check(models.Model):
 
     class Meta:
@@ -167,8 +172,8 @@ class Check(models.Model):
             result["tz"] = self.tz
 
         if self.last_ping:
-            result["last_ping"] = self.last_ping.replace(microsecond=0).isoformat()
-            result["next_ping"] = (self.last_ping + self.timeout).replace(microsecond=0).isoformat()
+            result["last_ping"] = isostring(self.last_ping)
+            result["next_ping"] = isostring(self.get_grace_start())
         else:
             result["last_ping"] = None
             result["next_ping"] = None
diff --git a/hc/api/tests/test_check_model.py b/hc/api/tests/test_check_model.py
index d34b8f55..b214b90f 100644
--- a/hc/api/tests/test_check_model.py
+++ b/hc/api/tests/test_check_model.py
@@ -74,3 +74,16 @@ class CheckModelTestCase(TestCase):
         # 11:30am
         now = dt + timedelta(days=1, minutes=90)
         self.assertEqual(check.get_status(now), "down")
+
+    def test_next_ping_with_cron_syntax(self):
+        dt = timezone.make_aware(datetime(2000, 1, 1), timezone=timezone.utc)
+
+        # Expect ping every round hour
+        check = Check()
+        check.kind = "cron"
+        check.schedule = "0 * * * *"
+        check.status = "up"
+        check.last_ping = dt
+
+        d = check.to_dict()
+        self.assertEqual(d["next_ping"], "2000-01-01T01:00:00+00:00")