Cat
Cat availability follows no schedule comprehensible to human logic. The animal may present itself for interaction, or may vanish into dimensional spaces that appear to exist only within domestic architecture. When sought, the cat is absent; when concentration is required, the cat materialises directly upon the keyboard. Availability is entirely cat-determined, and attempts to influence this scheduling are met with studied indifference. One does not summon a cat; one waits until the cat deems interaction acceptable. This may occur at three in the morning or not at all for days.
Sunset
The sunset operates on a schedule of astronomical precision. Its timing is predictable months in advance, with published tables available for any location on Earth. One need only face west at the appropriate hour to guarantee observation. However, this predictability is complicated by atmospheric conditions—cloud cover may obscure the event entirely, and the finest sunsets require specific meteorological circumstances that cannot be scheduled. The sunset is always occurring somewhere, yet meaningful observation depends upon factors beyond human control, including one's geographic position relative to buildings, mountains, and trees.