We did a similar thing, as we also offer time-based professional services. We work in a more project driven structure, so we created Projects as a custom object and allowed SF users to book time and travel expenses on these. Then we also created some reports which show us on a monthly basis which customer has to be billed how much for our services.

As this is a rather generic use case, we created an AppExchange app for this, which is completely free of charge:

Anybody who is interested can just install it. We would love to get some feedback :)