In an article on his blog Joel Spolsky discusses a system incorporated into FogBugz 6.0 that allows for “realistic scheduling”.

The idea is to provide a suite of tools to allow this to happen in a variety of platforms. Ideally a command line tool for task recording – so recording things is as easy as “start case 124” and “end case 214” on the command line – and a stand-along app for being able to produce reasonably realistic schedules.