Stelae

Louvre, No. 193

Serapeum Stela [1, p. 497]:

1.  Year 16, fourth month of the first season (fourth month), day 16, under the majesty of Horns: Wise-hearted (Śyʾ-yb); King of Upper and Lower Egypt; Favorite of the Two Goddesses: Triumphant; Golden Horus: Beloved-of-the-Gods;
2.  Uhemibre; Son of Re, of his body, his beloved: Necho, living forever, beloved of Apis, son of Osiris.
3.  The day of the interment of this god. This god was conducted in peace to the necropolis, to let him assume his place in his temple in the western desert of
4.  "Life-of-the-Two-Lands," after there had been done for him all that is done in the pure house, as it was done formerly. He was born
5.  in the year 53, second month of the second season (sixth month), day 19, under the majesty of the King of Upper and Lower Egypt: Wahibre; Son of Re: Psamtik (I), triumphant.
6.  He was [rece]ived into the house of Ptah in the year 54, third month of the first season (third month), day 12. He departed from life
7.  [in the year] 16, second month of the first season (second month), day 6. The total of his length of life was 16 years, 7 months, and 17 days. The majesty of
8.  the King of Upper and Lower Egypt, Necho, living forever, made all the coffins and every thing excellent and profitable for this august god.
9.  He built for him his place in the necropolis, of fine limestone of Ayan, of excellent workmanship. Never happened the like
10. since the beginning. That he might be given all life, stability, satisfaction, health, and joy of heart, like Re, forever and ever.

Lines 4-5 - Birth: He was born in the year 53, second month of the second season (sixth month), day 19, under ... Psamtik

Lines 6-7 - Death: He departed from life [in the year] 16, second month of the first season (second month), day 6 ... Necho

Line 7 - Lifespan: The total of his length of life was 16 years, 7 months, and 17 days.

This would indicate that Psamtik ruled an even number of complete years, but we cannot suppose that Psamtik I died on the last day of the year; it is evident that he died in the fifty-fifth year of his reign, and that the fraction of that incomplete year was, after his death, included in the first year of his successor, Necho. It is thus clear that the years of the king's reign in the Twenty-sixth Dynasty began on New Year's Day.

Therefore Psamtik I ruled for 54 complete years, before being succeeded by Necho in the 55th year.

Louvre, No. 240

Serapeum Stela [1, p. 501]:

1. Year 12, second month of the third season (tenth month), day 21, under the majesty of King Apries^ (Ḥꜥꜥ-yb-Re),
2. beloved of Apis, son of Osiris. The god was conducted in peace to the Beautiful West
3. to let him assum'e his place in the necropolis, the western desert of Memphis; after there had been done for him all that is done in
4. the pure house. Never was done the like before. The majesty of this god went forth to heaven
5. in the year 12, fourth month of the second season (eighth month), day 12. He was born in the year 16, second month of the first season (second month), day 7, under the majesty of
6. King Necho, living forever. He was installed in the house of Ptah
7. in the year I, third month of the third season (eleventh month), day 9, under the majesty of Psamtik II.
8. The beautiful life of this god, was 17 years, 6 months, 5 days. The Good God, Wahibre (Apries), made all the coffins,
9. everything, excellent, and every profitable thing, for this august god; that he (the king) might be given life and health therefore, forever.

Line 6 - Birth: He was born in the year 16, second month of the first season (second month), day 7, under the majesty of King Necho

Line 5 - Death: went forth to heaven in the year 12 [Apries], fourth month of the second season (eighth month), day 12.

Line 8 - Lifespan: The beautiful life of this god, was 17 years, 6 months, 5 days.

The combined, reigns of Necho and Psamtik II, therefore, lasted exactly twenty-one years.

Mortuary Stelae of the Priest Psamtik

[1, p. 518]

Year I, third month of the third season, day I, under the majesty of the King of Upper and Lower Egypt, Uhemibre (Wḥm-yb-Rꜥ), Son of Re, Necho (N-kʾw). On this good day was born the divine father, Psamtik, begotten of Ahuben (Y ꜥ ḥ-wbn), born of Enkhetesi (ꜥ nḫs). His good life was 65 years, 10 months, 2 days. Year 27, fourth month of the second season, day 28, was his day of departure from life. He was introduced into the Good House, and he spent 42 days under the hand of Anubis, lord of Tazoser. He was conducted in peace to the Beautiful West in the first month of the third season (ninth month), day ⸢—⸣; and his life in the necropolis is forever and ever.

Birth: Year I, third month of the third season, day I, under ... Necho

Death: Year 27 [Amasis], fourth month of the second season, day 28

Age: 65 years, 10 months, 2 days

hence, Necho's accession was exactly forty years before that of Amasis.

References

[1] J. H. Breasted, Ancient records of Egypt, Historical documents from the earliest times to the Persian conquest, vol. 4. University of Chicago Press, 1906, [Online]. Available: https://archive.org/details/cu31924082479258.