Add "in" method

src/index.js

@@ -1,8 +1,11 @@
 const isPast = require('date-fns/is_past');
+const differenceInMilliseconds = require('date-fns/difference_in_milliseconds');
 const addSeconds = require('date-fns/add_seconds');
 
 const expired = headers => isPast(expired.on(headers));
 
+expired.in = headers => differenceInMilliseconds(expired.on(headers), new Date());
+
 expired.on = headers => {
 	const originDate = new Date(headers.date);