HELP: Time format prob in excel 2007

Trying to do a sum( of time spent at telemarketing campaigns.
In order to calulate a true sum ,for several hundered campaigns, I'm entering
the total time spent in h:mm:ss and multiplying for the cost(€) per hour.
This works until the amount of time spent has 5 digits in the hours (ie, 12926:44:00).
As soon as I have a time like that I get an #value! error in the cell where the multiplication is done.
I have noticed that in the cells where the time has less than 5 digits in the hours,
the format is a date/time even though that's not how I set it up or how I'm entering the data
(ie, I enter 7715:46:00, but 11/16/1900 11:46:00 AM appears in the formula bar).
Help. I need this for a report due ASAP and been unable to find an answer anywhere.