When is this income considered earned

[Mike Landry]

My wife is a newer Realtor.  She earned a commission on the purchase of OUR house that closed on December 30, 2016.  However because it was our house she forgot to pick the check up at closing...it was a busy day.  So the title company mailed it to her and she received it on january, 4 2017.  When is the income considered earned/received according to the IRS?  It doesn't matter to me which year it goes in but it might make a diffence if tax laws change in 2017. Thanks!!

[Steven Hamilton II]

The day the check was written and available for her which means it was 2016.