Rather than trying to trim the frequency (which should not be that far off (see Robs post) by a coapacitor, let the RTC interrupt your uC every second (it probably does that already) and trim by a (long/double) counter that is decremented in that interrupt. When that counter read zero, reload it and increment or decrement your reading by a second. This method has the added advantage that you can calculete the load value of the counter, whereas with a trimmer you have to turn and fimd out how much or how little you turned wrong.

Erik