The Gantt chart working time (duration) is based on days.
The Gantt chart is based on durations set in days. You can define these working days in work calendars for your account.
The account default work calendar is 7 days e.g. if you set an item in the Gantt to have a duration of 7 days and a start date that fell on a Monday, the end date would be set to the following Monday, based on 7 working days for that item.
You can also set your own work calendars that define the working days you want to use e.g. you can create a 5 day calendar with the working days set to Monday to Friday (Saturday and Sunday excluded). Items in the Gantt that are set to use this work calendar will then use these days when calculating their end date; so in this case, if an item has a 3 day duration and a start day of Friday, it’s end day will be Wednesday, not Monday as Saturday and Sunday are non-working days in the work calendar.