I’m in the middle of writing a short story that rather prominently features time dilation as a central plot device.
Turns out time dilation is a complex little blighter. That’s why it took somebody like Einstein to nail down the concept in the first place.
Anyway, while researching I stumbled across a bunch of time dilation calculators. One of the simplest was this one: http://www.1728.org/reltivty.htm
Go check it out. It’s fun.