When people pay back support, do they pay interest on it? Or is it calculated in today's dollars?

Or is not paying like an interest-free loan?

Back support does not collect interest. If it did it would count as extra income that would have to be reported to the IRS by the party collecting the interest and the paying parent could write it off on their taxes just as interest on any other "loan". This is true in most states that I have had to deal with.

It depends on where you live. Calif charges 10% interest on arrears. They have a money making machine going.. lol