By default, the Task Due Date is based on the current date. So if the value is 2, the due date is two (working) days after the Tasks prerequisites (if any) have been completed.
This feature allows to calculate backwards.
Examples are a Release or a Meeting preparations to meet a target date.
With a negative relative date, the Due Date is calculated based on the Target Date.
In the Checklist Item, use a negative value for the Date Offset Days.
You can mix positive (based on the current date) and negative values in the checklist.
Optionally, you can default the Target Date from the record. In the Checklist, set the Dynamic Target Date - a reference to the date field (use the api name like MyDate__c)
- example: Use the Clise Date in the Opportunity as the Target Date
When creating the Tasks from the Checklist, enter the target date.
When you change the Target Date, the Due Dates are updated based on the Date Offset.
The date is stored in the Checklist Instance and you can update there, if required.