'Date From' and 'Date To' determine the payroll dates for salary purposes, while 'Post From' and 'Post To' determine the period for which working hours are considered, which include leave and overtime.
Let's say that you are doing a monthly payroll for August, on the 21st of the month. This means that any leave, sick leave or overtime dated after the 21st are usually unknown.
Therefore the 'Post From' and 'Post To' should be 22nd July - 21st August. 'Date From' and 'Date To' will still be 1st August - 31st August.
The next payroll's 'Post From' should then start from the 22nd of August. Hence, the dates are consecutive.