You mean why they are putting it off for a whole week and a half!? I don't think anyone would get their taxes done on time if it applied retroactively to 2017,

You mean why they are putting it off for a whole week and a half!? I don't think anyone would get their taxes done on time if it applied retroactively to 2017, and having a different tax code for December 21-31 seems like a bad idea also. I think applying the change on January 1st is ambitious if anything.