I taken over responsibility for a `Symfony2` application, built on the `Sonata Admin Bundle`, and have been asked to make a small change by the users. In the xls export of a list page, the dates all appear as e.g. `Wed, 01 Aug 2012 00:00:00 +0200`, but the Excel format is `General`. The users would like the data in this column to be an Excel date type, so that it is sort-able. 

I have been able to find some information about export customization, but this mostly concerns choosing the list export file types, or which fields to include, rather than how to change the format in the exported document. A similar question was asked [here](https://stackoverflow.com/questions/18586336/sonata-admin-custom-template) (I think) but there is no answer.

I think this would (or should) be very simple, but it is certainly not obvious. Any help would be much appreciated.