The date timezone is lost when exporting a document

If you export a document from a GMT system with datetime "8am" and import it in a GMT+10 system the datetime will still display as "8am" but this time according to GMT+10.

This also causes an inconsistency between the dates stored in the database and the date stored in the XML used to store document history. It's generally not much of a problem until you decide to change the system timezone which is the issue behind XWIKI-17445 (the timezone used to generate the data is not always the same as the one used at runtime).