Modified buildDateTimeColumn to automatically adjust for 1904 windowing

Previously, the buildDateTimeColumn method used DateUtil.getJavaDate() to read numbers as dates. However, this presented a problem when the spreadsheet was using 1904 windowing (using 1904 as the earliest possible date, which is standard on some Mac versions of Excel). See https://poi.apache.org/apidocs/org/apache/poi/ss/usermodel/DateUtil.html#getJavaDate(double)

Now, the method uses getDateCellValue(), which automatically adjusts for 1904 or 1900 windowing. See http://poi.apache.org/apidocs/org/apache/poi/ss/usermodel/Cell.html#getDateCellValue()