A lot of our dates and activities relate to three terms in the school year.
The start and end dates of the terms vary from year to year.
In reports I need to be able to identify an activity as falling within one of the five following timeframes:

before last term
last term
this term
next term
after next term

Each of these time frames will have a start date and end date
Where can I store these dates so that they can be accessed in formulae in custom fields?

Ideally I want just one copy that all custom fields and reports can reference so that when I update them at the start of each term every report automatically uses the new values.