Dear Barbara,
There is no grace period unless it is provided for in the court order for support.
It is late if not paid on the fist of the month, meaning it is late on the 2d day of the month, meaning 2d business day.
You can start to charge interest starting when the debt is 30 days late, effectively accrued from day one.
YOUR PAYMENT AND BONUS IF ANY MAY BE TAX DEDUCTIBLE

Edward M. Johnson