In PeopleHR, in a user's planner under Sickness, it will always show x days sick out of x working days.
The number shown for working days will show according to the user's work pattern and this doesn't reflect the number of sick days the user had in that holiday year.
The number of sick days the user had in that holiday year will be displayed beside days sick.
To add sickness for an employee on desktop, and for more information, see this article.