Add a live clock to Excel and change values accordingly

Hi!

I'm really new to excel and I want to do something a little
complicated.

I want to place a live clock on a spreadsheet and also display some
values (from another sheet) which would change dependent on what time
it is. I would also like a clock that counts down if possible.

Can anyone set me off on the right foot? I'm guessing I need to set a
function that constantly loops and checks the time, changing a value if
necessary.

Thanks a lot for your help in advance.