uploadArchives consequently uploads archives with a timestamp two hours behind

We build our projects with gradle 1.5 using Jenkins and use the uploadArchives task. This works fine but for some reason the timestamp generated is two hours behind:

:MyProject:uploadArchivesUploading: ....-1.0.0-20130523.182759-259.war to repository remote at http://xxx/artifactory/libs-snapshot-local
Transferring 20949K from remote
Uploaded 20949K

The build on the master started at 20:25 but for some reason the timestamp above is two hours behind.

I have checked that all our (Ubuntu) build slaves + master have the correct time. What does Gradle use to generate the timestamp in the artifact before uploading it to eg. artifactory?

Is the user.timezone property of the (forked) jvm perhaps set to UTC?

user.timezone is correct on each slave