Actually, Sylrae, I believe you're referring to
Arcane Age: Empire of Netheril, which did have a specialized spell (the name escapes me at the moment) for travelling through time to Netheril specifically, noting that other attempts to time travel to Netheril would always be made to fail by sending them back too far or not far enough.
Beyond that, however, time travel was (compared to the restrictions on traveling through time to Netheril) fairly lax in the setting. This got some coverage in some of the 3E web articles on time travel in the Realms, most specifically
Perilous Gateways: The Portal Through Time Part 2 and
Part 3 (the other parts of the series can be found on the menu on the left of those pages), and the web reprinting/revision of the
Chronomancer appendix
Chronomancy and the Multiverse.
Hmm. Looking it up; the spell is Time Conduit (Page 3 Cormanthyr Empire of Elves). When Mystryl was around (pre-fall of netheril), it was the only means of time travel, and all other methods become that if youre doing anything with that time period.
The 4 Time Gates use that spell.
After mystra comes along though, it doesnt say what happens, so maybe it opens up a bit, though I don't remember that being a thing. But it would appear it is. So it's not just Netheril, its any time period before Netheril Fell.
[Edit]Nevermind, Looks like I was right afterall.
"After the Year of Sundered Webs (-339 DR), Mystra simply maintained the old restrictions on time travel inherent in the time conduits and focused more on magic. Therefore, while the goddess is different, Mystryl's rules and restrictions on time travel remain enforced even 17 centuries after her destruction."
Same book, page 7.
So: Mystryl limited all time travel to follow all of the restrictions of her *ONE* spell, -Time Conduit-. And created 4 mythical Time-Gates that use it.
When Mystryl Died, Mystra took over, and simply continued to limit time travel as it had been limited before her.
Pre-Death of Mystra in 1385DR and the sundering of the weave, all time travel methods are also subject to the limits of the 9th level spell
Time Conduit, as detailed in Cormanthyr, Empire of Elves.
I would really love for them to collect and compile the setting lore, and not release retcons and contradictions all over the place. I hate contradictions and retcons. The more of them you have in your new books, the less I will like them - and I will ignore all of the contradictions and retcons in favor of the earlier printed material. Some stuff you can get away with, but not direct contradictions. Drives me crazy.
[Edit 2] Actually, at this point?
Dont write more crap. Distill the stuff thats been written. Compile useful books from all your previously published materials. Have people comb through all the FR Sourcebooks and Novels. Include all the major setting events that happened in the novels in the timelines. Include the events of Baldur's Gate, Icewind Dale, and Neverwinter Nights, since they've been hinted at in several other sources as being canonical. Collect the bits of lore you have scattered over dozens of sourcebooks. Where you contradicted yourself, don't choose one of them, present both options, and point out the contradiction so the GM can choose which version he wants to use.
Give me a Forgotten Realms Setting Cyclopedia. Perhaps complete with one-time purchasable Encarta-Like software with more purchaseable expansion packs.
Ideally keep the ages in separate books, and set them up with at least the option to ignore retcons and contradictions (put them in sidebars to mark them as optional if you feel the need to include them).
Arcane Age
Pre-Time of Troubles
Pre-Sundering
Post Sundering/Post Spellplague.
If you'd like to make this (pretty damn massive) task easier, get help from the Candlekeep forumgoers, of which many know more about the setting than the vast majority of your authors do.