If you read in the app a wrong value for Sunset or Sunrise, compared to your local congregation or some other source, it might indicate that your app is configured to take Elevation (Altitude) into account whereas your comparison source does not.
A few hundred meters Elevation might account for a difference of one to several minutes in the time of perceived sunset by the observer (when standing on a hilltop, one can see the Sun a bit longer above the horizon line, than one standing at sea level), for simple geometrical reasons. However, unless for mountain locations at high altitude, this parameter is generally ignored in most congregations.
While the GPS detection provides an accurate elevation measurement, you might want to set it to zero anyway. You can do this in the Geo Location Details screen in your app:
Click Edit on your current location:
Then, erase the Elevation field:
