When Does "Jubei-chan: The Secret of the Lovely Eyepatch" (1999) Take Place?
"Raburii Gantai no Himitsu" (released to English-speaking audiences as "The Secret of the Lovely Eyepatch" and later considered part of a two-part franchise "Jūbei-chan: The Ninja Girl") is a fantasy animated television series showran? by Akitaro Daichi that premiered April 5th, 1999. So when is "The Secret of the Lovely Eyepatch" set?
It takes place in the year:
We know this because Koinosuke Odago says to Jiyu Nanohana, "300 years... try imagining how long that is, Jiyu. Every person lives through a short life. People live doing what they believe is right, so they won't have any regrets. But, sometimes a person loses themself into the flow of time. It's not easy to realize this by yourself. There are people here who haven't realized it, and have been wandering, lost for the past 300 years." Yagyu Jubei (based on Yagyū Jūbē Mitsuyoshi 1607-1650)'s magic eyepatch passes to Jiyu Nanohana early on in episode one "Birth of the 2nd Yagyu Jubei" and Yagyu Jubei's death occurs during the 1650 AD prologue. So it is currently 1950? AD.

1950? AD
We know this because Koinosuke Odago says to Jiyu Nanohana, "300 years... try imagining how long that is, Jiyu. Every person lives through a short life. People live doing what they believe is right, so they won't have any regrets. But, sometimes a person loses themself into the flow of time. It's not easy to realize this by yourself. There are people here who haven't realized it, and have been wandering, lost for the past 300 years." Yagyu Jubei (based on Yagyū Jūbē Mitsuyoshi 1607-1650)'s magic eyepatch passes to Jiyu Nanohana early on in episode one "Birth of the 2nd Yagyu Jubei" and Yagyu Jubei's death occurs during the 1650 AD prologue. So it is currently 1950? AD.
1,650 + 300 = 1,950.
Additionally, there is a prologue at teh start of epsiode one "Birth of the 2nd Yagyu Jubei" depicting the death of samurai master Yagyu Jubei and the real-world Yagyū Jūbē Mitsuyoshi died on April 21st, 1650 AD. So it must be April of 1650 AD during the prologue.
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