Date transformation for KNN to get distance
What do you consider a meaningful distance? One possibility would be days since some reference date. 

If seasonality also matters, you could additionally (or also) have calendar date or some sine or cosine functions with period 1 year/n for integer n - the idea being to get a periodic spline. The same kind of idea (or flagging weekday vs. weekend etc.) could apply to day of the week, if that makes sense for your problem.