 It may be difficult to see where simultaneity came into the Doppler shift calculation, and indeed the calculation is often preferred because one does not have to worry about simultaneity.As seen above, the ship twin can convert his received Doppler-shifted rate to a slower rate of the clock of the distant clock for both red and blue images.If he ignores simultaneity, he might say his twin was aging at the reduced rate throughout the journey and therefore should be younger than he is.