Kaga said
"So wait... if he's from the 51st century, how is his name 'Jack'? You'd think that by the 51st century people would be all like, 'Hey, let's name our kids weird things like 'Ireland' and 'Orange Jello'!'"
The general idea is correct; even the oldest names we still use have changed in spelling and pronunciation since their inception.
For instance Henry would never have been considered a viable girl's name in the past and now it is.
Using the example of Henry again and goggling its origins, it apparently has its origins in the German name Heimirich (meaning home ruler) which then became spelled as Heinrich. The name was then exported over to France as Henricus (based on the Latin version of the meaning of Heimirich) and then shortened to Henri, at this point over a number of years it was exported to England where it eventually became Henry (and Harry apparently has its origins here to).
And that completely ignores the evolution of female variants of all the names above.
So over a number of centuries from around the 9th Century to the end of the Middle Ages , Heimirich became Henry/Harry. You'd imagine the difference between the 20th and 51st century would be even bigger in terms of names.