The time between 8:41 a.m. ET (when VK announced departure time) and 11:59 p.m. ET (the time he'd leave) is:

15 hours, 18 minutes, and 0 second
15/51=17(Q)x3
18=R=JFK Jr.

15.3 hours
15+3=18=R

918 minutes
9+1+8=18=R

55,080 seconds
5+5+8=18=R
55=5:5=pilot's term for "loud & clear"

😯
Does his next mission have something to do with JFK Jr.?
(Some think he is JFK Jr., but not so sure about that. But wouldn't it be a nice twist in this story?)