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The idea is to create a div within the entire body of the site for the user to view when entering, so it would have something like "Like the page to enter now or wait 30 seconds". The part of time you can do in js with setinterval and getelement to give a display:One after 30 seconds, the problem is the like part, how to do and how not to display the message again if the user has already liked.
What back-end language are you using?
– LeAndrade
I don’t think Facebook allows this kind of behavior anymore. It was very "popular" a few years ago and eventually the Facebook policy on this changed. Any solution you use without confirming with Facebook if the user gave like, it is easy to overcome. You would have to have an app submitted and approved with Facebook to be able to "ask" if the user gave like, but if you are approved and eventually reported, you will be blocked.
– Leite