Hi there, pegasus74!
It's a delight to see you in the Community. Allow me to jump in here and add up some details to the information provided by @Pete_Mc.
Are you referring to the Tax Cuts and Jobs Act tax law changes? If so, most of the changes in the new law will take effect in 2018, but will affect the tax returns by 2019.
You can find more information from these IRS articles:
I agree with @Pete_Mc above! It'd be best to consult with an accountant or contact the IRS helpline for the exact filing and payment dates. I can provide the general overview about this, while they have detailed information about its implementation.
These resources should help clear things up regarding this matter.
Don't hesitate to let me know if you're referring to something different. I'll be around, anytime!