russell - 2014-01-10

There is a time-fields flag in which you can specify the date source. The default is "from,received,date" which looks at the mbox "From " line first then at the "Received: " and "Date: " message headers.

In your case the mbox file has incorrect dates so you'll want to specify

--time-fields 'received,date'

or any one of those. Gmail respects the date the client sets so this will work.