Refactor expire function to return expirey date rather than time in ms

package.json

@@ -27,6 +27,7 @@
   "homepage": "https://github.com/lukechilds/onionoo-node-client",
   "dependencies": {
     "cache-manager": "^2.2.0",
+    "date-fns": "^1.21.1",
     "got": "^6.3.0"
   },
   "devDependencies": {

src/index.js

@@ -2,6 +2,7 @@ const got = require('got')
 const cacheManager = require('cache-manager')
 const querystring = require('querystring')
 const pkg = require('../package.json')
+const addSeconds = require('date-fns/add_seconds')
 
 class Onionoo {
 
@@ -34,21 +35,16 @@ class Onionoo {
     }, {})
   }
 
-  // Returns cache max age from response headers
-  calculateResponseMaxAge (headers) {
-    // Get max age ms
-    const cacheControl = headers['cache-control']
-    const maxAgeRegex = /max-age=(\d+)/
-    let maxAge = cacheControl && cacheControl.match(maxAgeRegex)
-    maxAge = maxAge ? maxAge[1] : 0
+  // Returns expirey date from response headers
+  expires (headers) {
+    const originDate = new Date(headers.date)
 
-    // Take current age into account
-    if (headers.age) {
-      maxAge -= headers.age
-    }
+    // Get max age ms
+    let maxAge = headers['cache-control'] && headers['cache-control'].match(/max-age=(\d+)/)
+    maxAge = parseInt(maxAge ? maxAge[1] : 0)
 
-    // Don't return negative values
-    return Math.max(0, maxAge)
+    // Calculate expirey date
+    return addSeconds(new Date(originDate), maxAge)
   }
 
   // Returns a function to make requests to a given endpoint