To change the task date and time
Clicking the drop-down button in the Actual Start and Actual End field opens the calendar, and lets you set the Actual Start/End dates.
Clicking the drop-down button in the Planned Start and Planned End field opens the calendar, and lets you set the Planned Start / End dates.
Use the left and right arrow buttons at the top of the calendar to move backwards and forwards a month, respectively, then click on the day you require.