Excel Report Builder does not allow you to format date fields. This is because of the way Excel will reformat them when using the report.
By default, when using Excel Report Builder, date fields will be formatted without any formatting which makes them a string field. To be able to format them as a date field, you have to do calculations on them, run a macro to convert them, etc.
In Excel Report Builder version 12.00.0067 (and higher) for Microsoft Dynamics GP 2013 and any of the releases for Microsoft Dynamics GP 2015, you can add a line to the Dex.ini to remove all date formatting from published Excel Reports. Add EXCEL_REPORT_BUILDER_REMOVE_DATE_FORMATS=TRUE to the Dex.ini to pass in a number rather than a formatted date to Excel. When you publish your Excel Report with this switch in place, all date fields will show as numbers similar to the Birth Date in the image below.