From my father A"H:
The posuk (Breishis 21 -2 ) says
“Lamoed Asher Diber Oso Elokim”
The Malach made a scratch on the wall and said that he will return
the following year, when the sun will be at the same spot, and Soroh
will have a child.
The malochim visited Avrohom the first day of Pesach.
Yitzchok was born the following year on the first day of Pesach.
The timing is off by 11 days!
It takes 365 days for the sun to return to the same spot . ( solar year).
From Pesach to Pesach the following year is only 354 days.
When the sun returned to the same spot it was 11 days after Yitzchok’s birth.
The Malach did not return for Yitzchok’s birth. He returned 11 days later.
It is written in the seforim that Malach Refoel visits newborn boys 3 days after their bris .(gimmel lemiloso) to heal them.
It was Malach Refoel who scratched the wall and promised to return exactly on the day when the sun will be at the same spot.
Malach Refoel returned to heal Yitzchok 3 days after his bris
Yitzchock was born 354 days after the besura (Pesach-Pesach) plus 8 daysfor the bris and 3 days (354+8+3=365)