If you get a visa well before you visit a country, will your visa expire by the time you visit the country?

I have never gotten a visa before so that's why I don't know. Basically I am wondering, when you get a visa do the days start expiring right when you get it, or when you enter the country? So let's say I'm from the U.S, and I'm in Egypt. If I want to get a 30 day Sudan visa in Cairo, but don't plan on going to Sudan until 15 days after I get the visa, by the time I enter Sudan, will my visa only be valid for 15 more days?

Submitted April 27, 2020 at 11:06PM by whyismylife_16 https://ift.tt/3eWua2K