Invalid Last-Modified header

After 2 months of use Wildfly suddenly started to send an invalid Last-Modified header. Normally it should be in the format:
Fri, 10 Oct 2014 19:35:55 GMT
But sometimes it sends:
Fri, 10 Oct 2014 21:35:55 CEST
Or even:
Fri, 10 Oct 2014 21:35:55 CEST GMT
The URLConnection.getLastModified() method cannot handle these 2 last formats as it uses an old and deprecated Date.parse(..) method.
A restart of the service fixes the issue for a few hours after which it reappears but not for all requests. We switched back to JBoss 7.1.1 and the issue disappeared completely so we're pretty sure that it's a bug in WildFly. No updates were done to windows, java and Wildfly that could have triggered the issue. We noticed the issue because we have a java applet running at the customer side. We use URLConnection to download some files based on the last modification date of these files. Browsers don't seem to be affected by the issue.