Swift 3 Calendar.range - migrate function
<p>I am not sure how to migrate this function correctly to swift 3.</p>

<pre><code>extension Date {  

    func numberOfDaysUntilDateTime(toDateTime: NSDate, calendar:NSCalendar) -&gt; Int {  

        var fromDate: NSDate?, toDate: NSDate?  

        calendar.rangeOfUnit(.Day, startDate: &amp;fromDate, interval: nil, forDate: self)  
        calendar.rangeOfUnit(.Day, startDate: &amp;toDate, interval: nil, forDate: toDateTime)  

        let difference = calendar.components(.Day, fromDate: fromDate!, toDate: toDate!, options: [])  

        return difference.day  

    }  

}  
</code></pre>