When does my iphone guarantee expire?

The iPhone does not have a guarantee, it has a warranty, and it is a limited 1-year warranty. However, user damage is not covered under warranty, that includes screen or other glass breakage. You can take it in for service, but it would be an out of warranty replacement device, then cannot replace the glass on the backs of the new phones. Cost depends on your location, but you can start here, iPhone Repair - Official Apple Support