Changing the Ad set's end time

That's certainly possible if you add those ad sets to your campaign after it's been published. However, your campaign end date will always show the date of the ad set end time that is furthest in the future. If none of your ad sets have an end date, then the campaign won't have one either. 

To know what your campaign end date is, go to the campaign report. There you'll see the duration of your campaign.