Before the end of the year, just to anticipate how much more you might owe, or get a general idea of any refund coming? You take into account all that is known, then make qualified conjectures on what is expected by year's end regarding income, deductions, etc. For the most part you are not far off the mark then when the end of year comes, especially if doing this about a month before that year's end. What's then required? You have to get the full report of your income and taxes paid to have the exact figures, which may differ by a few hundred, if even that. Corporations do this all the time, keeping a constant tally of known but also expected expenses while moving toward end of quarter and end of fiscal year.
Yes, not all changes in the bill was made, but corporations were already adjusting for possibility of it's passage. They would have done figures based on it's failure and it's passage. Once it was passed those figures could be quickly updated to reflect any changes that occurred. This goes on all the time and shouldn't be surprise.
Having those figures, once the bill was passed, it would take only a short while to firm them up and announce them in a timely fashion, which they did.