countdown timer that flashes/blinks

[Tinker B]

Hi All,

In VBA, I want to create a button where it cell 1B will countdown from 12 hours. Once it counts down to 00:00, I want cell A1 to start flashing/ blinking. I want it to continue flashing until I press a button called "stop". Could you folks point me in the right directions?

Thank you much,
Tinker B