By default, time is set in hours (worklogs, availabilities...) but you can change it.
3 possible cases to do so :
CASE 1 : SETTINGS
Go to your PROFILE then SETTINGS
Then CUSTOMIZE > TIME and check DISPLAY TIME IN DAYS.
CASE2 : ON YOUR CURRENT PROJECT
If your project is already set in hours, you can activate the time module in days.
Go to your project then tab FEES > MODULES > check TIME IN DAYS module.
CAS 3 : BY CREATING A PROJECT
(Click here to know more on the creation of a project)
Once you are at the FEES step, you can chose the fee calculation method in TIME.
Chose Time in days.
ADD ROLES TO PROJECT then SELECT THE PROJECT ROLES.
Set the daily billable rate, CLOSE then NEXT.
Put the working days by role et by deliverable then NEXT.
You will have the fees details of your project, by role and by deliverable. Then continue your project creation.