Time dilation refers to an effect of special relativity.

​It happens when time moves differently for a static observer than for a traveler in motion.

​The difference will be more profound the closer the traveler moves to the speed of light.

A video from BBC Four explaining a simplified version of time dilation. 
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Click below to access the Time Dilation Calculator, created by Bogna Szyk for Omni Calculator, last updated April 2024. 

Time dilation has created an age gap between twin brothers and astronauts Scott and Mark Kelly. Click the image of the brothers to access a Space.com article that explains how.

ADDITIONAL RESOURCES​

NASA Tumblr
"Bend Your Mind with Special Relativity"
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Time dilation explained
by Professor Antonio Paris for Planetary Sciences, Inc., including observations from the International Space Station and implications for interstellar flight.
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Encyclopedia Britannica 
​An article, including "60 Second Adventures in Astronomy" video from the Open University.
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Time Dilation
An explanation and exercises from Physics, LibreText.