When an admin enters a time off request, the system will automatically calculate the number of days based on parameters within the system.
In the event that you need to take less or more time than the time the system indicates (a half day, for example) it is possible to override the days calculated by the system simply by changing the days value manually once you have calculated the duration and changing the number of days accordingly.
If an employee is using self service, by default, they do not have the option of entering the number of days so the only option is for them to enter the details in the comments box so the approving manger can modify the request once submitted - we have implemented it in this way to stop employees erroneously or maliciously changing the number of days the manager either forgetting to or simply miscalculating the correct number of days which would then result in the deduction being incorrect.
As an alternative, you can allow employees to change the number of days before submitting a request by setting "Allow employees to enter timeoff days/hours" to Yes under Administration > Company > Settings > Time off settings (Global) under the 'Self Service' section.
However, please be aware, if you do this the system cannot validate the number of days being entered by the employee so an employee could choose start date of 03/01/2016 and an end date of 04/01/2016 and then edit the days to 5, for example, and it will be down to the vigilance of the approver to ensure the quantity entered is correct.
As a check, if the number of days entered by the employee is different to the number of days the system would calculate this is flagged as a warning when approving time off requests along with the number of days the system believes the request should be for but it is still down to the approver to approve/decline accordingly so diligence is required.