Tip for making the docstring a bit more clear to random users: this should say exactly what the model is used for.  Right now it basically just duplicates information, because the definition of what a model is will be clear from the `astropy.modeling.core.Model` link above.  So maybe for this part of the docstring, you could say something a bit more specific like "The Model that will be used to fix the timing data - the value input to the model will be the size of the timed function's input, and the output should be time in seconds." (I'm not 100% sure that's actually what the input and output are supposed to be, which just goes to show why it should be here :)

Similarly, for the fitter below, you could say something like "The Fitter object that will be used to fit the model to the timing data".  That tells you what it *does*, rather than what it *is*.