"I have time in the afternoon."
Translation:Mam czas po południu.
Accusative is "czas", Genitive is "czasu".
"mieć" takes Accusative, but when a verb needing Accusative is negated, then it takes Genitive.
Alternatively, you may have seen something like "Mam trochę czasu" (I have some time), "trochę" and other quantifiers like that need Genitive.