Fixed Dates is a task constraint users can use when establishing tasks on projects.
When users select the Fixed Dates (FIXT) task constraint, users must specify the precise start and end dates in the Planned Start Date and Planned Completion Date fields. Predecessor relationships will not force this task to reschedule its dates, it ignores the relationship.
The duration field of the task will not be editable, as it will be calculated from the start and completion dates you input.
Users should not use the fixed dates constraint with an effort driven task. Workfront will not be able to calculate a planned duration when the dates are fixed.