Because of the nature of the meeting, did it HAVE to be in Frankfurt or could it have been scheduled anywhere?
They were there to meet a certain person. That person was not there, though. They were in Paris.
All the relevant clocks watches etc at the time were accurate and would show the same now.
reply to cheeses_of_nazareth:
Would knowing the identities of the arrivees, or the third person they met, help solve the puzzle? Yes
Are any of them famous? (say, enough to have their own Wikipedia page?) Yes
Did the go to Frankfurt for some kind of competitive purpose? No
Did they go to commemorate some event? No
That’s where they expected the third party to be, so I guess.
Did the arrival take place before 12:01 a.m., January 1, 1950?
Would the participants have had any reason to be secretive about what they were doing?
reply to dirtball:
Did the arrival take place before 12:01 a.m., January 1, 1950? No (except according to one source 
)
Would the participants have had any reason to be secretive about what they were doing? No.
If they went when they did in fact go, then they would be on time. If they went now, they would be seven minutes late. So,
Is there any time between when they went and now when they would have been late by some amount less than seven minutes?
Alternately, was there some abrupt cut-off moment such that, if they had gone before that moment, they’d be on time, but after that moment, they’d be seven minutes late?
Backing up a bit…
Could the amount of lateness, in other but similar situations, be greater than seven minutes?
Could it be greater than 15 minutes?
Could it be greater than 30 minutes?
Could it be as large as an hour?
Could it be less than seven minutes?
Could it have been an arbitrarily-small time?
Were they astronauts?
reply to Chronos:
If they went when they did in fact go, then they would be on time. If they went now, they would be seven minutes late. And the certain other factors were the same!So,
Is there any time between when they went and now when they would have been late by some amount less than seven minutes? If the other factors were exactly the same then they would be late by (approximately) seven minutes.
Alternately, was there some abrupt cut-off moment such that, if they had gone before that moment, they’d be on time, but after that moment, they’d be seven minutes late? No
Backing up a bit…
Could the amount of lateness, in other but similar situations, be greater than seven minutes? It could easily be so now.
Could it be greater than 15 minutes? Sure
Could it be greater than 30 minutes? Yes
Could it be as large as an hour? Yes
Could it be less than seven minutes? Yes
Could it have been an arbitrarily-small time?
Yes
No
Let me see if I can summarize:
Two twentieth-century men intended to be at a particular place at a particular time.
They made plans to do so, using their own personal timepieces, which were probably wristwatches (but might have been some other sort of timepiece).
Their timepieces operated accurately and in the manner that they expected, and so they did, in fact, arrive at the planned place at the planned time.
However, if they were to make similar plans, under similar circumstances, now, their personal timepieces would, unbeknownst to them, be running approximately 7 minutes late, and therefore, if they relied upon those timepieces, they themselves would also be late.
Is this connected to the redefinition of the second, which took place in 1967 in Paris?
reply to Chronos:
Two twentieth-century men intended to be at a particular place at a particular time. Men? I do not think I ever said that.
They made plans to do so, using their own personal timepieces, which were probably wristwatches (but might have been some other sort of timepiece). No. They did not themselves have or consult any relevant timepieces.
Their timepieces operated accurately and in the manner that they expected, and so they did, in fact, arrive at the planned place at the planned time. They arrived when they were expected to arrive-- on schedule. This timeliness was assured thanks to certain wristwatches.
However, if they were to make similar plans, under similar circumstances, now, their personal timepieces would, unbeknownst to them, be running approximately 7 minutes late, and therefore, if they relied upon those timepieces, they themselves would also be late.
No. If they were to be at that certain place at that certain time and other events occurred the same today, they would be 7 minutes late.
Non
This is tricky- the motivation of the two arrivals in Frankfurt is still unknown, as is their connection to the third party-
Were the two people, at the moment they arrived at the Frankfurt location, aware that their arrival was at a ‘scheduled time’ in some sense? (or had they simply intended to arrive at a certain location?)
If the two were not aware, then was anyone else aware of the scheduled time of their arrival?
Would the two people, or others, have known they had arrived at the ‘appointed time’- however that is meant- or would that only have been apparent in hindsight?
Was their arrival in Frankfurt scheduled to coincide with some other event, rather than happening at a specific time?
Had the two people arrived in Frankfurt specifically to meet with the third person? And that person was in fact at that moment in Paris? Was the third person aware of the planned meeting in Frankfurt? Did the third person know the two people?
Were the people who were arriving intending to deliver something?
Pick something up?
Conduct negotiations of some sort?
Does it have anything to do with the fact that there are two Frankfurts in Germany, Frankfurt am Main and Frankfurt Main? In any case which Frankfurt is it?
Does it have anything to do with the fact that Frankfurt was in East Germany at the time, but is now in Germany proper?
Does it have anything to do with a change in the Polish-German border?
reply to cheeses_of_nazareth:
This is tricky- the motivation of the two arrivals in Frankfurt is still unknown, as is their connection to the third party-
Were the two people, at the moment they arrived at the Frankfurt location, aware that their arrival was at a ‘scheduled time’ in some sense? (or had they simply intended to arrive at a certain location?) They simply intended to arrive.
If the two were not aware, then was anyone else aware of the scheduled time of their arrival? Lots of people were aware of their scheduled time of arrival.
Would the two people, or others, have known they had arrived at the ‘appointed time’- however that is meant- or would that only have been apparent in hindsight? They were not aware at the time. Later, their timely arrival was noticed by many.
Was their arrival in Frankfurt scheduled to coincide with some other event, rather than happening at a specific time? It was at a scheduled time and NOT intended to coincide with another event.
Had the two people arrived in Frankfurt specifically to meet with the third person? YesAnd that person was in fact at that moment in Paris? Yes Was the third person aware of the planned meeting in Frankfurt? No Did the third person know the two people? Yes.