Final answer:
Trey can enroll in his PDP plan until July 31st, which is the same day his employer coverage ends. The effective date of the PDP plan depends on the specific plan's enrollment policies.
Step-by-step explanation:
The last date possible for Trey to enroll in his PDP plan would be July 31st, the same day his employer coverage ends.
As for when the PDP plan would take effect, it would depend on the specific PDP plan he chooses and its enrollment policies. Typically, PDP plans have a specific enrollment period during which individuals can sign up, and the coverage would usually start on the first day of the month following enrollment. So if Trey enrolls in his chosen PDP plan before July 31st, his coverage would likely start on August 1st along with his Medicare Part B coverage.