text: Title: Datetime export not working correctly from pandas
  Body: Using version 0.6.5 with Pandas 0.15.2 I'm using to_excel to export some dataframes which have a datatime variable as the first column.  I noticed the the exported spreadsheet values only contained daily values when I was expecting hourly values.  I traced to problem to the utility.py module and the  datetime_to_excel_datetime function.  Please check the equation at line 630 it appears to me to be missing delta.hours (and delta.minutes?) portion of the datetime value.  
  
  Thanks for providing this excel interface,  I have tried others and xlsxwriter fulfills my requirements the best.
  