professional services (consultants) resource allocation system
Google "open source project management software" - I got 49 million hits. There appear to be interesting options here, here and here.

There is very little that is truly original in software. What I would advise you to do is to download (a few of the) projects which look interesting, then check out their data models - that's the beauty of Open Source - look at the ideas of others and then adapt them to your own particular situation.

Many of the projects appear to be right up your alley, with many time assignment and/or management options which appear to be the core of your question.