The ornamental design for a golfballclock, as shown and described.
FIG. 1 is a front view of a golf ball clock showing my new design;
FIG. 2 is a rear view thereof; and,
FIG. 3 is a top perspective view thereof, the bottom, right and left side views being substantially identical thereto.
The structural elements depicted by broken lines have been provided for the purpose of illustrating portions of the golf ball clock that form no part of the claimed design.