asprung;
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Forget 'time is passing', and think 'events are happening throughout the universe'.
The rate of activity for the traveler in his spacecan, his clock, and everything that moves with him is slower because of his motion. Why, because all interactions at the atomic, molecular, and biological level are mediated by em/light exchange, and the speed of light propagation in space/vacuum is constant.
When he reads the slow running clock, his slow running memory can't detect it. Similar to 'length contraction',...if his spacecan did shrink 50%, he couldn't tell with his measuring rod which is 50% shorter. It's a matter of scaling, the difference being the clock events (ticks) are fewer and farther apart.
If his clock intervals are longer, and he sees the same number of events happening in the outside world, then his clock intervals contain more events than those of his earth-twin.
The same number of events happen for both, the traveller just partitions his 'time' into bigger pieces. Both persons remain in the 'now' world because there is no where else to be!