Time calculations on a 24-hr clock

Hello,

I need a formula that I can apply to my spreadsheet that will calculate how much time it takes someone to complete a task. Everything is based on start time (HH:MM:SS) and end time (HH:MM:SS). I currently have =(L2-K2) which is great but doesn't take into accont if the start time and end time are on different days. How do I fix this? Many thanks in advance for your help.