Frederick W. Taylor was a bastard, but it is helpful to keep track of the amount of time you spend on an activity if you're doing it for your own benefit. timestudy is here to help you in this endeavor.
timestudy intends to be an activity tracker similar to Timewarrior. It is mostly a project to help me expand my Rust skills, so you probably actually want Timewarrior, not this. (There are far fewer features at the moment, and probably far more bugs.) But perhaps someday it will be a decent alternative. I'm hoping to eventually build both a CLI (tm) and a GUI, but focusing on the library at the start.