[3.13] gh-123370: Fix the canvas not clearing after running turtledemo.clock (gh-123457) (#125653)

[3.13] gh-123370: Fix the canvas not clearing after running turtledemo.clock (GH-123457)

Rewriting the day and date every tick somehow prevented them from being removed either by clicking STOP or loading another example.  The solution is to rewrite them only when they change.
(cherry picked from commit c124577ebe915a00de4033c0f7fa7c47621d79e0)

Co-authored-by: Wulian <xiguawulian@gmail.com>