The ornamental design for a timer, as shown and described.
FIG. 1 is a front elevational view of a timer showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a bottom plan view;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a top, right front perspective view thereof.
The structural features depicted by broken lines have been shown for the purpose of illustrating portions of the timer that form no part of the claimed design.