Travel dates in I-GTM work on the principle of an academic year with holidays. A holiday has one or more outbound and inbound travel dates. An academic year has one or more start travel dates and end dates.
For example, a school might require all Upper VI and new pupils to arrive one day earlier than the rest of the school. Both of these dates are official school travel days and both are associated with the start of term.
The video below shows how to set up the academic year and add holidays and travel dates.