@Tianape, thanks for your clarification.
You’re right, but is something that is in Twitter’s hands, they decided to define that, if something is wrong coding you’re request, you will get an unauthorized_request. I think the real explanation is: If you send an URL with PARAMS, but those params are wrong coded (as dates you said) they encoded it with the oauth, and after that, when they decode that request maybe get something strange, and that’s the reason you get unauthorized, I don’t really know it. Is only a guess.
On the other hand, which library are you using? I’m quite sure that’s the reason. 'Cause, how do they know your time offset if you don’t provide it as an ISO 8601?
In my case we work with “YYYY-MM-DD” in my side, but when we make a request to twitter we transform that date into an standard.
Hope this help you
Regards!