How will TurboTax know? Because you tell TurboTax in the 1099-R interview that the money was a rollover. It is up to you to know if it was within 60 days from the date of the distribution. TurboTax does not know or ask for dates so the year does not matter. 60 days is 60 days even if the distribution was on December 29th. (The 5498 also will not show the date - only that it was a rollover if the IRA custodian deposited it within 60 days of the distribution.)
By the time that the IRS does any matching of 2016 tax returns (sometime late 2018 or 2019) they will have the 2017 5498 form, as will you. In any event if ever questioned, you should keep the receipt of other paperwork received from the rollover contribution that shows the date of deposit as your separate proof - your quarterly brokerage statement should also show the date of deposit as additional proof.
This is not anything to worry about as long as it was within 60 days.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
