diff --git a/package.json b/package.json
index 9329c98..3dc956a 100644
--- a/package.json
+++ b/package.json
@@ -14,6 +14,7 @@
     "bluebird": "^3.5.0",
     "elliptic": "^6.4.0",
     "jsonwebtoken": "^7.4.1",
-    "jwk-to-pem": "^1.2.6"
+    "jwk-to-pem": "^1.2.6",
+    "ms": "^2.0.0"
   }
 }
diff --git a/rest.js b/rest.js
index 9384e23..6c96ea1 100644
--- a/rest.js
+++ b/rest.js
@@ -8,6 +8,22 @@ function makeB64UrlSafe(b64) {
   return b64.replace(/\+/g, '-').replace(/\//g, '_').replace(/=*$/, '');
 }
 
+function timespan(duration, max) {
+  var timestamp = Math.floor(Date.now() / 1000);
+
+  if (typeof duration === 'string') {
+    duration = Math.floor(require('ms')(duration) / 1000);
+  }
+  if (typeof duration !== 'number') {
+    return timestamp;
+  }
+
+  if (max && timestamp+duration > max) {
+    return max - timestamp;
+  }
+  return duration;
+}
+
 module.exports.create = function (bigconf, deps, app) {
   var Jwks = { restful: {} };
   var Grants = { restful: {} };
@@ -445,16 +461,18 @@ module.exports.create = function (bigconf, deps, app) {
             kid: jwk.kid
           }
         };
+        var accessOpts  = {expiresIn: timespan(req.body.exp || '1d', token_info.exp)};
+        var refreshOpts = {expiresIn: timespan(req.body.refresh_exp, token_info.exp)};
 
         var jwt = require('jsonwebtoken');
         var result = {};
         result.scope = token_info.scope;
-        result.access_token = jwt.sign(payload, pem, Object.assign({expiresIn: req.body.exp || '1d'}, opts));
+        result.access_token = jwt.sign(payload, pem, Object.assign(accessOpts, opts));
         if (req.body.refresh_token) {
           if (token_info.refresh_token) {
             result.refresh_token = token_info.refresh_token;
           } else {
-            result.refresh_token = jwt.sign(payload, pem, Object.assign({expiresIn: req.body.refresh_exp}, opts));
+            result.refresh_token = jwt.sign(payload, pem, Object.assign(refreshOpts, opts));
           }
         }
         return result;