Of the four dimensions (Up-down, left-right, ahead-behind, before-after)Certainly by Newton's time were considered mathematically. Time is perpendicular to the other three, as are each of them to the others. This can't be visualized but it can sure be worked with mathematically. One of the two books I used to teach myself Celestial Navigation (just in time for computers and satellites to make all that work unnecessary) had a section on air and space navigation. All four-dimensional stuff. (and all definitely an Excedrin headache, believe me. I kind of skimmed those parts.)
Amy good introduction to Relativity (caution, do NOT try to read after the first Bourbon) will help with the presentation of time as a spatial dimension.
Haddon's excellent novel The Curious Incident of the Dog in the Night-Time,
which I cannot recommend strongly enough, not only shows how to tesselate St. George's crosses (knowledge without which my life was incomplete) but also presents a very easy way to understand time as truly spatial.
But once you've eaten of that tree, then the math for "many dimensions" becomes easy enough to understand, though incredibly tedious to do.
I couldn't begin to tell you "where" time is. :)
That-a-way.
Time is perpendicular to the other three, as are each of them to the others. This can't be visualized but it can sure be worked with mathematically. Thanks for bringing back the horrors of multi-variable calculus. So much for my plans of being a math major. :)
And thanks for the tip on Haddon's novel. I have a book which I've never finished called "About Time" by Paul Davies. I have to keep re-reading over the same sections, so I've never finished it. :) But I like it anyway.