One way to see a consequence of the lack of precision of the dates is the following:

• have an application on the wiki with a date field with "empty is today"
• create a new entry
• save without filling in the date
• upload an attachment to this page
• diff history: the date field will appear as a diff (the milliseconds part).
  • on some circumstances this difference in history can create a conflict upon save (in the sense of XWIKI-175), and it's completely not accurate, but this is another concern.

What's important for the current ticket is that because of this current migration issue, the behaviour described above may look sporadic for a farm: it will happen if the wiki is "older" and it will not happen if the subwiki is more recent.