1099 R

Simply report the 2018 Form 1099-R on your 2018 tax return and in 2018 TurboTax indicate that you moved the money to another retirement account and the amount rolled over.  As long as the rollover was completed within 60 days, the fact that the rollover was not completed until early 2019 is irrelevant.  The IRS is knowledgeable of the fact that the Form 5498 showing receipt by an IRA of a rollover contribution might be the Form 5498 for the year following the year of the distribution because of such circumstances.

You have 60 days from the date you receive an IRA or retirement plan distribution to roll it over to another plan or IRA assuming it qualifies for rollover.  miss it by one day and the entire amount is taxable unless you can get the IRS to waive the 60 day requirement. 

in addition, was any tax withheld because if there was and you don't make it up,  that portion is taxable.    Normally on a non direct rollover 20% is withheld.   example $1,000 gross, and taxable, $200, withheld so you only have $800,   within the 60 days you must put $1,000 into another retirement account.  Any less and the difference is taxable. 

if the check was dated in 2018, report on 2018 return.  the 1099 is issued based on the date the check is issued not the date you receive it.  in step by step mode you get to a question about what you did with the money.  just click the item - moved to another retirement account.  then click on the line that says rolled overall or part depending on which applies.   if it was a 100% rollover then you should see a message that says you don't have to pay tax on the distribution.    if there was withholding, you get to claim it in 2018.

the concept is called constructive receipt.   if I'm an employer and issue a paycheck to an employee

on 12/31/2018  (it could even be on 12/15/2018) that they don't receive until the following year it goes on the 2018 W-2.