Simplify the following text for a Grade 4 native English-speaking person.
Original: This was rectified by following the observations of Clavius , and the calendar was changed when Gregory decreed that the day after Thursday , 4 October 1582 would be not Friday , 5 October , but Friday , 15 October 1582 .
Simplified: These days were skipped between 4 October 1582 and 15 October 1582 . This calendar is still used today .