Parent B is custodial.
There is no such thing in the Federal tax law as 50/50, split, or joint custody. The IRS only recognizes physical custody (which parent the child lived with the greater part, but over half, of the tax year. That parent is the custodial parent; the other parent is the noncustodial parent.) Because 2017 was not a leap year with 366 days, you cannot be "equal."
It literally means the person with 183 nights is the custodial parent.
I have litigated this MANY times and the IRS will go so far as to require counting the nights spent in each household - that person is the custodial parent for tax purposes. Count each and every night in case either parent did not take the expected nights. It is tedious but necessary.
The exception to the above rule is IF the divorce decree states alternating years. In that case, the parent that did have the most overnights will sign Form 8832 to allow the other parent to be the custodial parent.
