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Let's suppose two ships that cross in space at constant speed. We do not know their previous acceleration or velocity history. When they cross, they synchronize watches.

From that moment, if they decide to come together later to see the difference of their proper times, can we directly say that the one who accelerates to join will be the one who has a shorter time or is it necessary to know their previous history?

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You do not need to know their previous history. For both ships the elapsed time is simply the length of their trajectory in spacetime from their separation to their reunion, where the length is calculated using the Minkowski metric.

This length is always longest for a straight line, i.e. unaccelerated motion, so if one ship accelerates and the other doesn't the ship that accelerates will have the smaller elapsed time.

For more on this see What is time dilation really?

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