In a few years, it won't be working in those same browsers, too.
Being "only" a candidate recommendation is significant. Standards are built on implementation. No spec will be finalized until there are at least two but, in this case, there are umpteen.
calc() works in all modern, desktop browsers and has for quite some time. It even works in IE9+. So you might only need to adjust for IE8 and below though IE users are used to missing out and wouldn't notice anyway.