I just finished reading the one-shot Ao to Umibe no Nostalgia and remembered I love the trope of tragic time travel romance, or just time travel romance in general. I'm specifically looking for ones where being from a different time period is an issue in the characters' relationship, not just time-travel romance or a romance and time-travel in the same manga (like Orange or Boku Dake ga Inai Machi). Perhaps something like Red River too, but less smutty. The manga equivalent of the movie Somewhere in Time or Kate and Leopold, you know. 🙂 Historical eras are best, but modern and futuristic is okay too. Just no magical lands/fantasy world travel.

Some other things I've read/seen that fit this:
Golden Days
Nobunaga Concerto (well, I've only seen the anime and drama)
Jin (saw drama)

I don't want hentai or anything along those lines. And no incest. (Why does this seem to be a deal in time-travel manga? 🤣 ) Nothing graphic, please. But I'm okay with shoujo-ai and shounen-ai.

This isn't a manga, but what about the TV mini-series Lost in Austen? The heroine gets transported into Pride and Prejudice, which is kind of historical. Hilarious series.

There's also Queen In-Hyun's Man (K-drama) or Love Weaves Through a Millennium (C-Drama). Interestingly, I've heard the Chinese remake version is better, but I haven't seen both of them to compare.

There isn't a huge time difference, but I really recommend Kimi ni Shika Kikoenai. Here are some other time travel romances:
Shinobi Life
Fushigi Yuugi - Genbu Kaiden - not entirely historic, the MC is transported to a fictional historic setting though so I thought I'd just throw it out there.
The oneshots Kami-sama no Sonata & Mahoroba no Yogiboe
The animated movie "The Girl Who Leapt Through Time".

If you don't mind anime, you should really try Buddy Complex and its sequel. Pretty much exactly what you're looking for. It's only 15 episodes long, too.
Hi no Tori
Ryuurouden, maybe.
Yukarism -- Another 'maybe,' since I dropped it early on.
Heptagon
Gyakusou Shoujo - Owaranai Natsuyasumi
The second story in Concrete Garden might fit, but it's been ages since I read it, so I'm not sure whether romance played an important role or not.

Oh, but none of these are really tragic like Golden Days is; despite all the hardships, the couples in the above titles do end up together.

@SilverStorm

Queen In-Hyun's Man is my favorite k-drama! I didn't know there was a Chinese remake, though. I'll have to see if I can find it. I saw Lost in Austen back when it came out as well. Thank you, though.

@Teanao

Kimi ni Shika Kikoenai looks good, I'll have to give that a try. I knew about Shinobi Life but thought it looked a little silly (and long), but I guess it is pretty similar to Queen In-Hyun's Man, so I should probably read that one too. Fushigi Yuugi looks to be more like the fantasy world travel genre... I'll take a look at the oneshots as well.

@calstine

I'm not necessarily looking for tragic. 🙂 I don't normally like mecha but I'll take a look at Buddy Complex. Your other suggestions look interesting as well. My sister's been trying to get me to read Yukarism for ages now.

Have you seen RahXephon? While it's not obvious from the start, romance and time travel (of sorts) are key themes in that (very post-NGE) mecha anime.