The Trump administration announced a new policy Friday that tightens rules for foreign or exchange students who overstay U.S. visas by counting days overstayed, not from the time the violation is reported, but from the time the visa expires.

Why it matters: The new rules, which are set to go into effect in 90 days, will count significantly more days as time overstayed. Students are banned from re-entering the country based on how long they overstayed their visas. Per the Wall Street Journal, 180-day overstay results in a 3-year ban, and an overstay of over a year prompts a 10-year ban.

By the numbers: "Some experts have estimated that 40% or 50% of about 11 million undocumented immigrants in the U.S. came legally but stayed past their departure dates," reports the Journal. And international students, specifically, are among the largest group of foreign visitors who overstay their visas.