Time-Dependent Tax (TDT) is a new functionality that enables customers to create and maintain VAT tax code changes by themselves. Once the TDT functionality is rolled out for a country/region, all the existing customers of that country/region must activate TDT in order to maintain, change or create VAT tax codes. The VAT tax codes for these countries/regions can no longer be created or changed with the help of SAP Expert Configuration team. 

For more information about TDT, please refer to this document.