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. 

Click the image at left to access a Space.com article about how time dilation has created an age gap between twin brothers and astronauts Scott and Mark Kelly

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.