時代 and 世紀: Contextual times and numbered centuries
Read a grandfather’s youth and changing times beside an early twentieth-century station and a story’s twenty-third-century setting.
A grandfather’s youth and changing times
時代（じだい） can mean an era or times. 「この道具は祖父が若かった時代に使われた。」 means “This tool was used when my grandfather was young.” 祖父が若かった describes the time of use. It does not identify the grandfather as the person who used the tool.
「時代が変わっても残したい習慣がある。」 means “Some customs are worth preserving even as times change.” The Japanese 残したい expresses wanting to retain customs; the supplied English translation calls them worth preserving. Neither states that changing times require discarding every custom.
Time described by a numbered century
世紀（せいき） means century. 「この駅舎は二十世紀の初めに建てられた。」 means “This station was built in the early twentieth century.” 二十世紀の初め places the station’s construction in time.
「物語は二十三世紀の海上都市を舞台にしている。」 means “The story is set in a maritime city in the twenty-third century.” 二十三世紀 supplies the story’s time, while 海上都市 supplies its setting. A narrative setting does not report construction of a real city.
Read the contextual era and century number
時代 appears in the context of the grandfather’s youth or changes over time. 世紀 appears with a number identifying the time of a building or story.
These labels and numbers do not supply extra historical start or end years for a region. Keep customs worth preserving, construction of the station, and a story’s setting attached to their own subjects.
Does 物語は二十三世紀の海上都市を舞台にしている report a city actually built in the twenty-third century?
Show the answer and why
No. It identifies a twenty-third-century maritime city as the story’s setting. Distinguish that setting from the station’s construction in 二十世紀の初め.