machine learning on athlete performances to predict the time in a future race
I think the key here is what you believe to be dependent variables. You mentioned, for instance, a three month rest. Encoding the rest between races is likely to be a better idea than simply encoding the date of the rest as a lot of the date is redundant to what is actually being said. As with any machine learning algorithm, representation of the data is key and in many ways, the actual algorithm applied is less important.