Simplify the following text for a Grade 8 native English-speaking person.
Original: Later that day , it was upgraded to Tropical Storm Darby , and only four hours later , at 2 am ( 0900 UTC ) on July 27 , is upgraded to Hurricane Darby .
Simplified: Later that day , it was upgraded to Tropical Storm Darby , and was upgraded to a hurricane on July 28 .