At a distance of 4.2 light years, and a speed of 37,500 kilometers per second, the round trip elapsed Earth time assuming no deceleration or acceleration would be twice x ( 4.2 ly / 37,500 km/s) = 67.1 years.

The relativistic time dilation factor for ship's time would be 1/( 1- (1/8)^2)^1/2 = 1.008 so the duration of the trip in terms of the ship's clock would be 67.1/1.008 = 66.6 years. In other words, the travelers would return to Earth 67.1 - 66.6 = 0.5 years 'younger' than what the Earth clock says the round trip time took.

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