Prevent OverflowErrors from long time periods

A couple of different locations in the code were causing crashes if the
user specified ridiculously long time periods (via topic listings).

This should prevent it, or at least give a better error when it happens.

tildes/tildes/lib/datetime.py

@@ -18,7 +18,11 @@ class SimpleHoursPeriod:
             raise ValueError('Period must be at least 1 hour.')
 
         self.hours = hours
+
+        try:
             self.timedelta = timedelta(hours=hours)
+        except OverflowError:
+            raise ValueError('Time period is too large')
 
     @classmethod
     def from_short_form(cls, short_form: str) -> 'SimpleHoursPeriod':

tildes/tildes/models/topic/topic_query.py

@@ -121,7 +121,15 @@ class TopicQuery(PaginatedQuery):
 
     def inside_time_period(self, period: SimpleHoursPeriod) -> 'TopicQuery':
         """Restrict the topics to inside a time period (generative)."""
-        return self.filter(Topic.created_time > utc_now() - period.timedelta)
+        # if the time period is too long, this will crash by creating a
+        # datetime outside the valid range - catch that and just don't filter
+        # by time period at all if the range is that large
+        try:
+            start_time = utc_now() - period.timedelta
+        except OverflowError:
+            return self
+
+        return self.filter(Topic.created_time > start_time)
 
     def has_tag(self, tag: Ltree) -> 'TopicQuery':
         """Restrict the topics to ones with a specific tag (generative)."""