Why Do Doraemon and His Sister Dorami Live in Different Time Periods?

Hey, that's a great question! Many newcomers to Doraemon might wonder about this. The reasons involve both story setting and making the plot more interesting.

You can think of it like this:

Simply Put: One is on a "Long-Term Assignment," the Other is "Holding Down the Fort"

It's like one sibling being sent abroad for long-term work while the other stays in the hometown.

Doraemon's Mission is a "Long-Term Overseas Posting" His main task, or "job," is entrusted by Nobita's great-great-grandson, Sewashi. He traveled back to the 20th century specifically to care for and help Nobita, who was still an elementary school student at the time. His mission is to correct history and improve the future of the Nobi family. This is a very specific and long-term assignment, so he must stay by Nobita's side.
Dorami's "Primary Duties" are in the Future Dorami lives in 22nd-century Tokyo. Her main responsibility is taking care of Sewashi. She's essentially Sewashi's nanny robot. So, her "home" and "workplace" are both in the future.

Because they each have clearly defined responsibilities and specific people to care for, they naturally live in different eras.

Why Doraemon, Not the More Capable Dorami?

You might then ask: Since Dorami is clearly more capable, smarter, and more reliable, why wasn't she sent to help Nobita? Wouldn't that be more efficient?

This involves deeper story elements:

Performance and Cost Issues In the original setting, Doraemon was actually a "factory-reject" robot. He had minor defects during production (like missing a screw) and later had his ears bitten off by mice, making him terrified of them. Precisely because he was a "bargain-bin" robot, the financially struggling Sewashi family could afford him. Dorami, however, is his younger sister robot, manufactured later with superior performance and a much higher price tag. So, economically, Doraemon was the only feasible choice to send back to help Nobita.
Plot Necessity This is actually what makes the story so fun! Precisely because Doraemon himself is clumsy, a bit scatterbrained, and often pulls out gadgets that backfire, he and Nobita—who has his own flaws—can become true "partners in misfortune." They grow together and face difficulties together. If the perfect Dorami were sent instead, she'd probably solve all the problems in no time. Nobita wouldn't get the chance to grow, and the story would lose much of its humor and heartwarming moments.
"Visits" are Always Possible Even though they live in different eras, remember they have the Time Machine! Dorami can always "pop in" from the future to visit her brother at Nobita's house. She also steps in as a "substitute caretaker" when Doraemon malfunctions and needs to return to the future for repairs. This has become a common and very popular recurring element in Doraemon stories.

To Summarize

So, Doraemon and Dorami live in different eras mainly because:

Division of Duties: The older brother is responsible for "helping" his ancestor in the past, while the younger sister takes care of the descendant in the future.
Historical Reasons: The "affordable and practical" Doraemon was the one originally sent.
Narrative Tension: An imperfect robot helping an imperfect human makes for a much more compelling dynamic.

Hope this explanation clears things up! Though separated by time and space, the siblings share a very close bond!