If you want to find out how long a task has taken to complete, there are many ways to calculate difference between times in Excel. The ones I use most are:

  A B C
1 START TIME (A2) END TIME (B2)  
2 2/26/2013 10:45 AM 2/26/2013 5:30 PM Result
3 To calculate the difference in standard time: =TEXT(B2-A2,”h:mm:ss”) 6:45
4 To calculate the difference in hours: =INT(B2-A2)*24 6.75
5 To calculate the difference in minutes: =INT(B2-A2)*1440 405