Converting time to a number (00:00:00 to 0.00)

you should multiply 24 to time. example:
8:30-5:00=3:30
if you want to see this as 3.5. first put the format in number. then multiply by 24. it means:
3:30*24=3.5(number format)

note:
if you sum number of cell with time format,you should put the result cell in this format:
[hh]:mm:ss

Feel free to find more tutorials on my Excel Blog.