When you enter your Form 1099-R when Amending your 2017 Tax Return, the code "P" will say "taxable in 2016"; however, in the next step, you should be asked if this is a 2017 or 2018 1099-R. When you choose "2018", the year advances by one.
Note: Code "P" is always one year before the issue year on the 1099-R.
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