Deleting the Predecessor value on the Parent Row will remove this constraint and allow you to manipulate the child row dates individually.
If a row has a given Duration and Predecessor value, the Start and End dates will be calculated automatically.
Clicking the date-picker icon in the cell will ensure that the date is in the correct format.
If your sheet includes a Gantt chart, the Start and End dates will be used to plot out bars in the chart.
Doing so may change the Duration value and would break any dependencies (remove the Predecessor values) previously established.
Dragging from either end of the task bar will extend the Duration to a new Start or End date.