It is not true that he can claim her for 2016 when he files 2017. If he failed to claim her in 2016; he would have to file an amended 2016 return to do so. Or, if he did not file a 2016 return, he would have to do so now.
You should be aware of three things when each parent is claiming one child.
1. For tax purposes, there is no such thing as joint custody, regardless of what your legal agreement says. The requirement, to be custodial
parent, is
that the child live with you MORE than 50% of the time. One of you has to be
the custodial parent and the other the non-custodial parent. The IRS goes by physical custody, not legal
custody. Yes, the IRS expects you to count the numbers
of nights the child sleeps at each parent's home. It is allowed for you
to arrange the children's schedules so that one child spends more than half the
year with the father while the other spends more than half with the mother.
Then you are each the custodial parent of one child.
2.The
custodial parent has first priority on claiming the children on her taxes;
regardless of the amount of support provided by the non-custodial parent. The
non-custodial parent can only claim the child as a dependent if the custodial
parent gives permission (on form 8332) or if it's spelled out in a pre 2009
divorce decree. Even if a divorce decree, dated after 2008,
gives the non-custodial parent the right to claim the child, he must still get
form 8332 from the custodial parent. A properly worded decree should require
her to provide that form.
3. There is a
special rule in the case of divorced & separated (including never married) parents.
When the non-custodial parent is claiming the child as a
dependent/exemption/child tax credit; the custodial parent is still allowed to
claim the same child for Earned Income Credit, Head of Household filing status,
and day care credit. This "splitting of the child" is not available
to parents who lived together at any time during the last 6 months of the year;
then only one of you can claim the child for any tax reasons. The tax benefits
may not be split in any other manner.
Note in particular
that the non-custodial parent can never claim the Earned Income Credit, Head of
Household filing status or the day care credit, based on that child, even when the custodial parent has released
the exemption to him.
So, it's good idea to let the other parent know that you
will be claiming those items, as many first time divorced parents are not aware
of this rule and may try to claim those items, which will cause the IRS to send
out letters.
Ref: http://www.irs.gov/publications/p17/ch03.html#en_US_2014_publink1000170897
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