First post in a topic is created with the wrong date

Open ../xwiki/bin/edit/BBCode/TopicClassSheet?language=en&editor=wiki and add line with BBCode.PostClass_0_postdate element:

...
<input type="hidden" name="BBCode.PostClass_0_postauthor" value="$context.user" />
<input type="hidden" name="BBCode.PostClass_0_postdate" value="$datetool.get('yyyy/MM/dd HH:mm')"/>
<div class="bbbigbox">
...