when you say that the Baseline start and finish date is different from his computer. I assume you mean that the calculated start date and calculated finish dates are different NOT the START of the Project or the PROJECT MUST FINISH BY DATE.
You also say that "I already get the actual start and finish date right" if this is a baseline schedule then there should be NO progress meaning no actual start ot actual finish dates contained in the schedule.
Regardless once the actual start and or finish dates have been recorded then they are no longer considered during the schedulng calculations so it would be impossible for them to be incorrect on your computer so you do not need to worry about this.
You say that "We are using the same working hours per day,weeks, year" if you are refering to the settings in the Admin preference - Time period settings then there are only two thing that can be making the dates different.
ONE - you are using a default calendar that is different from the computer where the xer was exported from.
even if you are using the same default calendar name look at the deatils for calendar on the computer where the xer came from vs the deatils on your computer for your default calendar chances are they are different. You may have 2 extra holidays in your calendar than the default calendar on the computer where the xer came form. Go thru month by month from the start of the project and compair the calendars on a month by month bases to determine if they are the same.
TWO as Syed suggested look at which calendar for scheduling relationship is using and make sure that you are both using the same reatined logic setting when scheduling progress activities. Like Syed said press F9 then options and compaire these two setting also.
My guess is that you are using global default calendars not projetc specific calendars. I am 99% certain is that the calendars are different and that is your issue..