Since time is relative, it progresses as we experience as we are not traveling. But from the perspective of one traveling at near light speed, time progresses to them, as they would experience it as if they were not traveling. However, the difference between the two could be substantial, as we sit static, we observe the traveler, and the travelers clock seems to move slower than the static clock next to us.
Due to this, the static clock will tick "normally" for us, but the travelers clock we observe ticks slower. So an hour for us, passes as maybe only six minutes to the traveler. So we have aged an hour to the travelers six minutes. We are ahead of the travelers time by fifty four minutes. The traveler isn't moving forward in time, like in a Back to the Future type conundrum. We are. To us, time progresses in the same linear fashion as it always has. To the traveler, time progresses in the same linear fashion as it always has. We would both experience the passage of time similarly, but our experiences are relative to our position.
So if an hour passed for us, during the six minutes it passed for the traveller, then how much difference would there be if he travels for one year?
If I'm doing the math right, and he experiences time at one tenth speed (six minutes versus our sixty), then one year to him would be like 876 years for us. So one year of travel away from earth, at a constant speed, and one year of travel back at a constant speed, he has experienced two years passage of time, while those on earth have experienced 1752 years.