Use timezone-aware datetimes with croniter, avoid conversions to and from naive datetimes. This avoids ambiguities around DST transitions and properly solves #196

CHANGELOG.md

@@ -13,6 +13,7 @@ All notable changes to this project will be documented in this file.
 ### Bug Fixes
 - Fix after-login redirects (the "?next=" query parameter)
 - Update Check.status field when user edits timeout & grace settings
+- Use timezone-aware datetimes with croniter, avoid ambiguities around DST
 
 
 ## 1.3.0 - 2018-11-21

hc/api/models.py

@@ -15,6 +15,7 @@ from django.utils import timezone
 from hc.api import transports
 from hc.lib import emails
 import requests
+import pytz
 
 STATUSES = (
     ("up", "Up"),
@@ -115,12 +116,15 @@ class Check(models.Model):
         if self.kind == "simple":
             return self.last_ping + self.timeout
 
-        # The complex case, next ping is expected based on cron schedule
-        with timezone.override(self.tz):
-            last_naive = timezone.make_naive(self.last_ping)
-            it = croniter(self.schedule, last_naive)
-            next_naive = it.get_next(datetime)
-            return timezone.make_aware(next_naive, is_dst=True)
+        # The complex case, next ping is expected based on cron schedule.
+        # Don't convert to naive datetimes (and so avoid ambiguities around
+        # DST transitions).
+        # croniter does handle timezone-aware datetimes.
+
+        zone = pytz.timezone(self.tz)
+        last_local = timezone.localtime(self.last_ping, zone)
+        it = croniter(self.schedule, last_local)
+        return it.next(datetime)
 
     def get_status(self, now=None):
         """ Return "up" if the check is up or in grace, otherwise "down". """

hc/front/views.py

@@ -28,7 +28,7 @@ from hc.front.schemas import telegram_callback
 from hc.front.templatetags.hc_extras import (num_down_title, down_title,
                                              sortchecks)
 from hc.lib import jsonschema
-from pytz import all_timezones
+import pytz
 from pytz.exceptions import UnknownTimeZoneError
 import requests
 
@@ -130,7 +130,7 @@ def my_checks(request):
         "now": timezone.now(),
         "tags": pairs,
         "ping_endpoint": settings.PING_ENDPOINT,
-        "timezones": all_timezones,
+        "timezones": pytz.all_timezones,
         "num_available": request.team.check_limit - len(checks),
         "sort": request.profile.sort,
         "selected_tags": selected_tags,
@@ -325,13 +325,11 @@ def cron_preview(request):
     tz = request.POST.get("tz")
     ctx = {"tz": tz, "dates": []}
     try:
-        with timezone.override(tz):
-            now_naive = timezone.make_naive(timezone.now())
-            it = croniter(schedule, now_naive)
-            for i in range(0, 6):
-                naive = it.get_next(datetime)
-                aware = timezone.make_aware(naive, is_dst=True)
-                ctx["dates"].append((naive, aware))
+        zone = pytz.timezone(tz)
+        now_local = timezone.localtime(timezone.now(), zone)
+        it = croniter(schedule, now_local)
+        for i in range(0, 6):
+            ctx["dates"].append(it.get_next(datetime))
     except UnknownTimeZoneError:
         ctx["bad_tz"] = True
     except:
@@ -420,7 +418,7 @@ def details(request, code):
         "page": "details",
         "check": check,
         "channels": channels,
-        "timezones": all_timezones
+        "timezones": pytz.all_timezones
     }
 
     return render(request, "front/details.html", ctx)

requirements.txt

@@ -1,5 +1,5 @@
-croniter
-Django==2.1.3
+croniter==0.3.26
+Django==2.1.4
 django_compressor==2.2
 psycopg2==2.7.5
 pytz==2018.7

templates/front/cron_preview.html

@@ -11,11 +11,13 @@
 {% else %}
 <table id="cron-preview-table" class="table">
     <tr><th id="cron-preview-title" colspan="3">Expected Ping Dates</th></tr>
-    {% for naive, aware in dates %}
+    {% for d in dates %}
     <tr>
-        <td>{{ naive|date:"M j, H:i" }}</td>
-        <td>{{ aware|naturaltime }}</td>
-        <td class="hidden-xs">{{ aware|date:"c" }}</td>
+        {% timezone tz %}
+        <td>{{ d|date:"M j, H:i" }}</td>
+        {% endtimezone %}
+        <td>{{ d|naturaltime }}</td>
+        <td class="hidden-xs">{{ d|date:"c" }}</td>
     </tr>
     {% endfor %}
 </table>