If a holiday duration is greyed out, it indicates that the user was assigned to a work pattern during that period when the holiday is booked for.
When assigned to a work pattern, the duration information is picked up from the working hours specified in the work pattern.
To override this, assign the user to the No work pattern with an effective date before the holiday date.
To do this:
Click Access Button
then click Employee.
Select the relevant employee then click Planner.
Click on their work pattern.
From the drop-down list, select No work pattern.
Set the effective date to at least one day before the old work pattern.
Go to the relevant day and change the duration, then click Save.
Change the work pattern back to how it was set up initially.