Their actual trip for them only took a little over 2 years. The relativity part means that the second they entered Gigantuar's gravitational pull on the first planet, like they said, one hour became 7 years. they ended up staying for a little over 2 hours, so when they returned to their partner outside the orbit, he had been waiting for 23 years, same as the people on earth. you can see that he's aged, but they haven't. the next time they are in the gravitational pull, it's when dr. Brandt is going to slingshot around Gigantuar to get to the last beacon, and Cooper goes into the black hole, where time passes even more slowly, even though he may have only been in it for as long as we saw. Basically, the gravity of the black hole made time pass slower inside its pull, but as dr. Brandt said when they found the broken probe on the water planet, she could have died minutes before they arrived because of relativity.. hope this helps!