Display a field for a specific amount of time
I don't know what format your date field uses so you'll have to adjust for that, but something along the lines of 

if(($content['field_date'] &gt;= $first_date) &amp;&amp; ($content['field_date'] &lt;= $second_date)) {
  print render($content['field_date');
} else {
  print($wrong_date_message);
}


...where $first_date and $second_date are the beginning and end of the date window for which you want to display the field, and $wrong_date_message is whatever you want to show up there instead when the field isn't displayed.

For more information about date formats in PHP, check out http://php.net/date.