Export to Excel - how do/can I control the cell format

I have RDL reports using 2008 schema

I am exporting to excel

The format of the data does not appear in excel as I expect.

I have counts, percentages, and durations (hh:mm:ss) fields on my report

For numbers,  if I set the textbox properties to be a number with Regional settings enabled, the cell is formatted as a number Custom [$-10409]0.  

For durations, if I set the textbox properties to be time 13:30:00, the rendered report cell is formatted as General.

Any suggestions on how I can get the duration cells to show as hh:mm:ss in excel once exported?

(Automating Report Viewer 10 in my app to generate the report)

(My durations come out of Sql as time in seconds - an integer - then I run it through some code that converts it to hh:mm:ss)