Your w2 for 2018 can only report wages earned in 2018 unless you are using a non-calendar tax year (very unlikely).
However, you should note that it is based on when the wages are paid, not when the work is done.
You should treat these wages as earned in 2018 in every way, or else you need to get a corrected w2 from your employer. My recommendation--you're using the free edition, just report them as earned in 2018, even if you are "darned sure" they were earned in 2017. The main thing is you are reporting the wages earned in accordance with the 2018 w2 received, and that's tax compliance for the little guys like all of us.
