The ornamental design for a countdowncalendar, as shown and described.
FIG. 1 is a front perspective view of the countdown calendar showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof, the left side view being a mirror image of the right side view;
FIG. 5 is a top view thereof; and
FIG. 6 is a front perspective view of the countdown calendar in an open position;
FIG. 7 is another front perspective view of the countdown calendar;
FIG. 8 is a front view thereof;
FIG. 9 is a rear view thereof;
FIG. 10 is a right side view thereof;
FIG. 11 is a left side view thereof;
FIG. 12 is a top view thereof; and,
FIG. 13 is a bottom view thereof.
The features shown in broken lines with consistent dash lengths illustrate portions of the countdown calendar or environmental subject matter to the countdown calendar forming no part of the claimed design. The features shown in long dash-short dash bolded broken lines indicate the bounds of the claimed design and otherwise form no part of the claimed design.