As a Hard core Sci-Fi writer, I want to get the science, at least near, right for my readers. When it comes to space travel, this is where your seasoned, educated readers will pick your work apart. Yes, there is a certain artistic licensing an author will use to get through space and time, but your readers want you to have some sense or rightness.
So here are some calculators I have come across for my own work, which have helped me in my timelines.
http://www.easysurf.cc/cnver15.htm – This is a great site that contains all sorts of distance calculators from Astrological Units to Kilometers, light seconds to miles to Parsecs to light years.
http://www.1728.org/reltivty.htm – Relativity Calculator, usually referred to as Speed Dilation. The concept of time passage relative to the speed of light.
http://nathangeffen.webfactional.com/spacetravel/spacetravel.php – Space Travel Calculator (Takes some knowledge of Astrophysics)
http://www.calculatoredge.com/civil%20engg%20calculator/speed%20Distance%20Time.htm – Another set of speed calculators for practical speeds. This site also has calculators for a wide range of studies (i.e. Chemical, Physics, Optical, etc.).
This is just plain weirdly beautiful, if you are a visual kind of writer.