diff --git a/github/Repository.py b/github/Repository.py
index 98a57235..0c6c8702 100644
--- a/github/Repository.py
+++ b/github/Repository.py
@@ -983,7 +983,7 @@ class Repository(github.GithubObject.CompletableGithubObject):
         if description is not github.GithubObject.NotSet:
             post_parameters["description"] = description
         if due_on is not github.GithubObject.NotSet:
-            post_parameters["due_on"] = due_on.strftime("%Y-%m-%d")
+            post_parameters["due_on"] = due_on.strftime("%Y-%m-%dT%H:%M:%SZ")
         headers, data = self._requester.requestJsonAndCheck(
             "POST",
             self.url + "/milestones",
diff --git a/github/tests/ReplayData/Repository.testCreateMilestone.txt b/github/tests/ReplayData/Repository.testCreateMilestone.txt
index 79b5acc8..1897b3d5 100644
--- a/github/tests/ReplayData/Repository.testCreateMilestone.txt
+++ b/github/tests/ReplayData/Repository.testCreateMilestone.txt
@@ -4,7 +4,7 @@ api.github.com
 None
 /repos/jacquev6/PyGithub/milestones
 {'Content-Type': 'application/json', 'Authorization': 'Basic login_and_password_removed', 'User-Agent': 'PyGithub/Python'}
-{"due_on": "2012-06-15", "state": "open", "description": "Description created by PyGithub", "title": "Milestone created by PyGithub"}
+{"due_on": "2012-06-15T00:00:00Z", "state": "open", "description": "Description created by PyGithub", "title": "Milestone created by PyGithub"}
 201
 [('status', '201 Created'), ('x-ratelimit-remaining', '4958'), ('content-length', '604'), ('server', 'nginx/1.0.13'), ('connection', 'keep-alive'), ('x-ratelimit-limit', '5000'), ('etag', '"bb5eb08c923020c102396cd0c9bfdbc5"'), ('date', 'Sat, 19 May 2012 10:24:13 GMT'), ('content-type', 'application/json; charset=utf-8'), ('location', 'https://api.github.com/repos/jacquev6/PyGithub/milestones/5')]
 {"closed_issues":0,"url":"https://api.github.com/repos/jacquev6/PyGithub/milestones/5","due_on":"2012-06-15T07:00:00Z","creator":{"avatar_url":"https://secure.gravatar.com/avatar/b68de5ae38616c296fa345d2b9df2225?d=https://a248.e.akamai.net/assets.github.com%2Fimages%2Fgravatars%2Fgravatar-140.png","url":"https://api.github.com/users/jacquev6","gravatar_id":"b68de5ae38616c296fa345d2b9df2225","login":"jacquev6","id":327146},"number":5,"open_issues":0,"title":"Milestone created by PyGithub","created_at":"2012-05-19T10:24:13Z","state":"open","description":"Description created by PyGithub","id":121463}