Set countdown timer, then pop-up message when complete

As the title suggests, I want to be able to have two separate cells C14 and C15, where I can set countdown timers by entering the value in hours and minutes (HH:mm, for example at 3pm means 15:00) in each and receive a pop-up message when timer is complete. I want both cells to operate independently from each other and countdown on separate timers and receive separate pop-up messages.

So far I've had no luck finding the right code to pull this off... So I'm coming to you all for help