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.
