title: SOLR delta-import timestamp issue
body: <p>I'm new to SOLR and was doing some research on this technology. I now have a question regarding the delta-import function so I looked on SO and found this: <a href="https://stackoverflow.com/questions/4620774/solr-dataimporthandler-delta-import">Solr DataImportHandler delta import</a>. In the answer there is a field <code>[date_update]</code> mentioned which seems to be a timestamp of the record.</p>
  
  <p>My question is: Is [date_update] a timestamp stored in the table on record creation? If so, cannot this create an issues if the date of the Database Server is not exactly in sync with the server on which SOLR is installed? This could possible leave out some records if the Solr server time is ahead of SQLServer time.</p>