Time in Lieu

Time in Lieu

I would like to be able to keep a record of time in lieu for my employees. How can I keep track of time in lieu in total, and how can an employee mark a day as using said time?

For example, 

employee has worked >40hrs in a week. They want to mark anything over 40hrs as TOIL. 
Now the following month they would like to use some of that banked time to have a day off. 

Is this a possibility now? If so, how can I do this?