Oh-oh-oh! (and oh dear, this turned into a long post. sorry about that)
Feminine wa Utau isn't exactly time-travel, but it invovles traveling from present/reality into a sort of fantasy-world. And it's really good—a lot of fun, gorgeous art, interesting plot...I recommend it
(and if you like it, check out
Tenkuu Seiryuu and
Princess Syndrome by the same author—Tenkuu is set entirely in a fantasy world, while Princess goes between the present and a fairy-tale world).
That made me think of
Fusuma Land 4.5, which has more reality-to-fantasyland back and forth. It is a pretty dated art style, but a really sweet and funny story!
The series lialah was thinking of is
Tonari no Shugoshin. It's pretty and cute... rather silly, but fun. By
SHIMAKI Ako so if you're familiar with her you know about what to expect. More of a fantasy guy from the European "past" (he's the Lancelot of Arthurian legend) randomly popping up in the present, rather than the other way around, so it takes place all in the real/present world as far as I know.
Gekka no Kimi by the same author is a better series (in my opinion) and deals with the reincarnation of a character from the famous Tale of Genji (the world's first novel, set around 900AD-ish—Heian period in Japan). Scenes from both the past and present are used throughout the story.
Oyayubihime Infinity is interesting—it deals with reincarnation, and characters go through flashbacks of their past, set in the Japanese past (not sure about a specific date, but I'd estimate at least 1700s to maybe early 1900s). Licensed by CMX. I really really adore this series
I haven't read much of
Golden Days but it deals with a character from the present finding himself in the past. It seemed pretty interesting so far.
Okay, and this is getting pretty far from your theme (maybe) but
Sugar Sugar Rune deals with two witch girls from the Magic World living in the Human World (they go back and forth throughout the series). It's really glittery, all-out-shoujo style, but it's full of awesomeness if you look past the cutesycutesy exterior (and even that has a lot more style and fun and originality than generic-shoujo-cookie-cutter art). It's also licensed, so you'd have to pick up a physical copy somewhere to read it. But it's totally worth it, I think