You can create a 24/7 calendar to assign to tasks which should be calculated on elapsed or continuous time.
To do this:
1) Add a new calendar in Navigator>Calendars
2) Right click on any Saturday and select Normal Working Day>Every Saturday
3) Repeat step 2 for any Sunday
4) Right click on any day and select Edit Day Type
5) Set the start time to 00:00 and the end time to 24:00
6) Change the day description to 24 hour day
Tasks assigned this calendar will be scheduled on a continuous calendar. A duration of 1d will be calculated as 24 hours elapsed time.