Use PreparedStatement#setTimestamp() wherein you pass a java.sql.Timestamp which is constructed with System#currentTimeMillis().
preparedStatement.setTimestamp(index, new Timestamp(System.currentTimeMillis()));
// ...
Alternativaly, if the DB supports it, you could also call a DB specific function to set it with the current timestamp. For example MySQL supports now() for this. E.g.
String sql = "INSERT INTO user (email, creationdate) VALUES (?, now())";
Or if the DB supports it, change the field type to one which automatically sets the insert/update timestamp, such as TIMESTAMP instead of DATETIME in MySQL.