How do I create a countdown with local save?

Hey guys! I try to create a countdown and i want to save the result with local storage.

Here is what i try to make:

Imagine you open a game, and there is a button with bonus coins. So, when you press the button to get the coins and a text appear after like this: "Next coins: 02:00:00".

So we keep playing the game and we close it after 1 hour, whatever.

When we open the game again, text will appear something like this: "Next coins: 1:00:00" and the story goes on..

.. Until we open it again and appear the button with the new bonus coins.

Something like this.. Anyone can help me? Thanks!