终于让它工作了！不是很漂亮（我希望有一个更干净的解决方案），但是这样做有效：
>> Payment.all(:conditions =>
["Date((payments.created_at at time zone 'UTC')
at time zone :timezone) >= :start_date and
Date((payments.created_at at time zone 'UTC')
at time zone :timezone) <= :end_date",
:start_date => start_date, :end_date => end_date,
:timezone => 'Asia/Katmandu'])
不过，我并不真正喜欢这样做：
Date((payments.created_at at time zone 'UTC') at time zone 'Asia/Katmandu')
为何postgresql不允许您这样做？
Date(payments.created_at at 'Asia/Katmandu')
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