BUG: `days_at_time` should return UTC dates.
BUG: `days_at_time` should return UTC dates.

Adds an example and clarifies the docs.

Scapogo/zipline,enigmampc/catalyst,alphaBenj/zipline,florentchandelier/zipline,alphaBenj/zipline,humdings/zipline,quantopian/zipline,enigmampc/catalyst,bartosh/zipline,quantopian/zipline,florentchandelier/zipline,Scapogo/zipline,humdings/zipline,bartosh/zipline