I claim the ornamental design for a cooking timer, as shown and described.
FIG. 1 is a front elevational view of a cooking timer showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a right side elevational view, the left side being a mirror image thereof.
The structural features depicted by broken lines in FIG. 2 have been shown for the purpose of illustrating portions of the cooking timer that form no part of the claimed design.