How do you freelancers track your time and bill your users?

[Btaliat]

It is pertinent for the freelancers to know much on how to track their time and as well bill their users. This means they need to be making use of the right tool that can help in executing this. This will make it easier for them. There are sometimes that tracking of time in order to know the right amount to charge may be somehow difficult. Which tools are you using as freelancers to do this?