How the days are counted
The day you arrive counts as day 1. With a 90-day stay starting on 1 March, day 90 is 29 May, so you must leave by 29 May. The exact date is also shown on your entry stamp or in your e-arrival record; when in doubt, that date wins.
Residence card (ARC) deadline
If you came on a long-term visa and will stay more than 90 days, you must apply for an Alien Registration Card (외국인등록증, now called Residence Card) within 90 days of entry at your local immigration office. Book through HiKorea. Missing the deadline can mean a fine.
FAQ
Can I extend a visa-free stay?
Usually not, except for serious reasons such as illness or flight cancellation. Apply at an immigration office before your stay ends.
What happens if I overstay?
You may be fined and may have trouble entering Korea again. If you have already overstayed, contact the Immigration Contact Center (call 1345, many languages) before leaving.