I think this occurs when you create a time chart and export that data. I don't think you can add a field to a timechart just like that.
timechart count by domain,_time
returns _time not valid.
You can go to excel and convert the date manually.
So as an example you can use the datevalue or date functions in excel.
when 2012-11-30 is the first date of your splunk data
and your data has a span of an hour (24)
and column B in excel is a bunch of numbers starting from 0 and incrementing by 1 every row down.