Time format issues in pivot table

Hi,

I am compiling pivot table, to graph power usage on 30 minute increments. I am having issues with the time format in the pivot table, the source data is in hh:mm:ss format but when I pull the data into the pivot table, the 30 minute values seem to get split out. I have done the graphs I want, manually, but this is very time consuming and I am sure there must be a quicker way through the pivot tables.

If anyone can help, that would be great. EDIT abbreviated file now attached.

Jennifer

Can you post a smaller version of the file? We don't need all the data, just a representative sample.