[michaelerule]

Is the 6 years measured in the time-frame of a passenger on the spacecraft, or as observed from Earth?

[Udo]

At 0.9c the relativistic time dilation is such that 2.3 days on Earth pass for every day shipboard time, meaning the crew would experience less than 4 years of travel in total: https://www.fourmilab.ch/cship/timedial.html

[raxxorrax]

Since we aren't too fast yet, it would basically be the same.