In Streamtime, a default setting prevents time from being allocated on the weekend.
When this setting is active, allocated time on the weekend shows up as negative estimated time.
It will even drag this time into the working week to make up for the time that you had allocated on the weekend.
In order to allocate this time on the weekend, you'll need to make a change to Streamtime's settings.
Go to Setup > System Setup > Jobs and in the column on the left-hand side, you will see a setting 'Skip Weekends on Gantt'. If you are currently unable to allocate time on weekends, this will be set to 'Yes'.
Change this to 'No'.
You can now allocate time on weekends in the Task Calendar.