excel – elapsed time calculation over 24 hours

I am trying to show elapsed time in Excel which is very often over 24 hours. I want it to show only in hours, mins, (secs) I have seen some posts referring to using the time format of 37:30:55 but I cannot find this time format in my version of excel (2007). Am i looking in the wrong place or is there some way to use the custom format to make the spreadsheet do this. any advice welcome

Chris