BUG: Use Timedelta instead of DateOffset in days_at_time calendar helper

Prior to pandas 17, there were issues with offsetting dates with
DateOffset around discontinuities (like the start of DST). We can use
Timedelta here instead, which handles these edge cases.

zipline/utils/calendars/trading_calendar.py

@@ -21,7 +21,7 @@ from pandas import (
     DataFrame,
     date_range,
     DatetimeIndex,
-    DateOffset
+    Timedelta
 )
 from pandas.tseries.offsets import CustomBusinessDay
 
@@ -678,9 +678,9 @@ def days_at_time(days, t, tz, day_offset=0):
         The number of days we want to offset @days by
     """
     days = DatetimeIndex(days).tz_localize(None).tz_localize(tz)
-    days_offset = days + DateOffset(day_offset)
+    days_offset = days + Timedelta(days=day_offset)
     return days_offset.shift(
-        1, freq=DateOffset(hour=t.hour, minute=t.minute, second=t.second)
+        1, freq=Timedelta(hours=t.hour, minutes=t.minute, seconds=t.second)
     ).tz_convert('UTC')