When l want to sell an item, how and where do l go to set the timer clock for the item ?

[polly.the.cat]

How do l set the days, hours, & minutes left on an item that l want to sell, so l can see the time decreasing as the sale period finishes ?  

I am sure that l have clicked on something in the past, but for the life of me, l cannot find it now. Any help would be thankful.