The issuing consulate has given you permission to stay in the Schengen area for eight days. To allow some flexibility in flights they have allowed you to arrive on or after 17th July and depart on or before 8th August. So, whatever date you arrive, you must leave the area within eight days, or by 8th August, whichever comes first.
In your situation you have two choices: apply for a new visa to cover the additional period, or delay your arrival until 21st July, so that you can depart on 28th July.
Note that the day of arrival and day of departure each count as one day, even if you're only in the area for part of a day.