June 15 is 5 months plus 15/30 days = 5.5 months out of 12. The seller's share would be:
(5.5/12)($4800) = _____
The buyer pays for the remaining 6.5 months:
(6.5/12)($4800) = _______ (= $4800 - the seller's share).
A more precise method would be to count the days individually. In a non-leap year, June 15 is day #166 out of 365 (it'd be day 167 out of 366 in a leap year). The seller's share would be:
(166/365)($4800) = _______