local_storage_expiration.js

function getLocalStorage(name){ 
    var now = parseInt(new Date() / 1000);
    var expires = localStorage.getItem(name+"_expire");
    if(!expires){ return localStorage.getItem(name); }  
    else if(now >= expires){ localStorage.removeItem(name+"_expire"); localStorage.removeItem(name); return ""; }
    else{ return localStorage.getItem(name); }   
}

function setLocalStorage(name, value, minutes){ 
    if(minutes == null){ localStorage.setItem(name, value); localStorage.removeItem(name+"_expire"); return true} // No set expiration
    else if(minutes == 0){ localStorage.removeItem(name); localStorage.removeItem(name+"_expire"); return true} // Setting to 0 kills the localStorage Item any any expiration
    else{ 
        var epochExpire = parseInt(new Date() / 1000 + (minutes * 60));
        localStorage.setItem(name, value); localStorage.setItem(name+"_expire", epochExpire); 
        return true
    }
    return false
}

Updated to 'STRICT' javascript (IE variables are defined instead of assumed)