Summary:
When you export report data from Web TimeSheet to MS Excel, the output is always dependent on your MS Excel version. 
.XML file format for MS Excel XP and higher versions. 
.csv file format for MS Excel versions older than Excel XP.
Example:
When values like 01-2733 are exported to MS Excel 2010, it considers the data to be in date format and shows it as a date. This happens when Report Excel Export Format is set to .csv in Web TimeSheet Settings

Resolution:
In Web TimeSheet, click on Settings option available at the top right, beside the Logout button. Change Report Excel Export Format to .XML.