There are some special cases that are not simply matters of using the basic interest rates published for a tax jurisdiction.   The following special cases are implemented in Instant Interest:

• Federal tax-motivated underpayments
• Large underpayments ("hot interest") for California,
  Federal, and Virginia
• Large (in excess of $10,000) corporate overpayments for Alabama
  and Federal
• Colorado’s discount for early payment
• Delaware’s delayed interest accrual on refunds
• Florida’s tax amensty program
• Interest rates fixed based on Due Date for Florida, New York
  City, Utah
• Iowa’s use of a rounded monthly rate
• Missouri’s special handling of refunds for amended returns
• New Jersey’s lower interest on extensions granted or discretionary
  reductions
• Oregon’s additional 4% interest for delinquencies after 60 days
• Tennessee’s interest rate fixed based on the Assessment Date
• Texas’ 60 day grace period before beginning interest calculations
  
• 52 and 53 week taxpayer calculations