What is the exact reign years for Japanese Emperor Keiko, Emperor Seimu, Emperor Chuai, Empress Jingu and Emperor Nintoku?

Question

According to Wikipedia, although starting with Emperor Sujin, they are with a direct possibility of existence, they still presumed legendary until Empress Jingu. However, Kofun Culture (http://www.t-net.ne.jp/~keally/kofun.html) reference webpage said emperors starting with Emperor Keiko, they might be real (until Emperor Chuai). This webpage already corrected the reign time of Emperor Ojin as to 370 - 390, but the others provides only century period. Emperors after Emperor Nintoku (not including him) appear to be aligned with the actual reign century period mentioned there. What is the exact reign time year periods for these emperors?

Research effort:

Please note that the non-research data below comes from Wikipedia. I think the reign time of Empress Jingu should be 361 - 370, because late xth century generally refers to the 61th-100th or 71th-100th year, the first one is ok, and the second one is duplicated with reign time of Emperor Ojin. Emperor Nintoku's reign time should be 390 - 400, which is the blank of Ojin (r. 370 - 390) and Richu (r. 400 - 406). Because Emperor Seimu reigned around the first half of the 4th century, the reign time of Emperor Chuai should be 351 - 361 (which the year 351 is the start of the second half of the 4th century). Wikipedia said that both Keiko and Seimu reigned around the 4th century and the second one is recorded as the "first half of the 4th century" in Wikipedia. Due to lack of century period (early/mid/late), I can get reign time conclusion of neither Keiko nor Seimu.

Answer 1

The reason for the confusion is that we don't know the exact dates. We have traditional dates, and we have reconstructions, but the latter can be disputed.

Per the Cambridge History of Japan, vol. 1:

But our knowledge of what was really occurring between the rule in the third century of Queen Himiko (reported in the Wei chih) and the reign from 592 to 628 of Empress Suiko (who reestablished relations with China) continued to be quite hazy and imprecise.

If you want the traditional dates, Gina Barnes in her Protohistoric Yamato (1988) has them listed, along with better estimates for their actual rule:

Regent	Traditional Dates	Adjusted/True Reign Dates
Keiko	71–130	219–249
Seimu	131–190	249–280
Chuai	191–200	280–316
Jingo	201–269	316–343
Ojin	270–310	346–395
Nintoku	313–399	395–427

The reason for this is just the time that had since elapsed and imperfect reconstructions by ancient (now to us) Japanese scholars, as Barnes notes (p.10):

The compilation of the materials that were later incorporated into these chronicles began during Emperor Keitai's reign in the early sixth century. This reflects on the historicity of the texts: the Nihon shoki's early sections are chronologically unreliable, but events recorded from the fifth century onward are thought to be fairly accurate; Philippi (1969: 5) reports also that it "has often been suggested that the 'now' of the Kojiki refers to this period" under Emperor Keitai.

(Emphases mine.)