Most effective discount solution?

I used to have my website put products on sale certain days of the week, but am considering changing it so that it offers the discount for a certain amount of time after the visitor first goes to my website, is that a good idea?
I was thinking of making it offer a discount that expires either at midnight of the first day they visit or a fixed amount of time after their first visit such as 2 hours and display a live countdown showing when the discount will expire. I could have the discount come…
Most effective discount solution?

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