You can set the time frame for a Calendar-Based Task when you create or edit a task. See the types of time frames below.
Specific Time
When you set a specific time, you decide for the assignee of the task when to begin the task - at the "Start date" and "Start time" set by you.
Flexible Time Frame
When you set a flexible time frame, the assignee of the task decides for themselves when to begin and complete the task - anytime between the "Can start from" date and time and the "Must finish before" date and time set by you.