Thanks guys, didn't think this would be something covered by warranty. This is the one I've got:
http://www.canadiantire.ca/AST/brows....jsp?locale=en
Says there's a lifetime warranty so I'll look into that. In the meantime I'm thinking it's close enough to accurate for the purposes of putting my Summer tires back on safely until I can re-check with a more trusted tool?
Edit: Shit, calibration warranty is only 90 days... :(