First Alert ZCOMBO expiration of NOS units?

For those with experience owning ZCOMBO (1044807), how does the ‘end of life timer’ work? Is it like the hour meter on industrial equipment or is there some physical self-check? I assume these are a dumb device without access to a clock and/or constant power supply?

The reason I ask is because this model is discontinued and I see NOS with 2018 date codes. That’s technically expired even though new in sealed package. Will they beep right out of the box?