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In the episode "The Blind Banker", when Sherlock meets Sebastian, Sherlock looks at Sebastian's watch and deduces that he had made two international trips in one month because the time was right but the date was two days behind. I didn't get this part.

This doesn't make sense as his trip was to start and end at same place. Even if he had forgotten to alter his watch still it would be showing the current correct time and date. Also, in one international round trip we cross the dateline twice: first while going and second while returning.

Please explain.

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2 Answers 2

up vote 7 down vote accepted

Your reasoning makes sense if you take the same route back on which you came.

However, in Sebastian's case, he made a complete trip around the globe twice, so he crossed the dateline twice without crossing it back in the opposite direction. Meanwhile, he forgot to set his watch back.

JOHN: Two trips around the world this month. You didn’t ask his secretary; you said that just to irritate him.
(Sherlock smiles but doesn’t respond.)
JOHN: How did you know?
SHERLOCK: Did you see his watch?
(Brief flashback to Sherlock looking at Sebastian’s wrist as he scratched his neck.)
JOHN: His watch?
SHERLOCK: The time was right but the date was wrong. Said two days ago. Crossed the dateline twice but he didn’t alter it.

So he had to be travelling west across the International Date Line (IDL) twice:

Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day.

Why not travel the same route back east?

There can be several reasons:

EDIT:

The op correctly noticed that if Sebastian never changed the watch at all, it would show the correct date. The watch does not really care about travel or datelines, and time went on normally for the watch and London (relativity effects omitted), so without any change on the watch the date and time would be correct again after the travel. So he must have changed the time without changing the date, that's what lead to the problem. That is an easy mistake to make, because it is common knowledge that you have to adjust your watch to the local time, but you only ever have to adjust the date when you travel over the dateline.


Sherlock further deduced that it had to be within a month from the fact that the watch was relatively new:

JOHN: Within a month? How’d you get that part?
SHERLOCK: New Breitling.
(Flashback close-up on the watch showing its brand name: Breitling Chronometre Crosswind.)
SHERLOCK: Only came out this February.

Transcript source


Fun fact: Sherlock was wrong about the last part, the model of the watch has been out for years:

The one thing Sherlock got wrong! [..] The model featured is a Breitling Crosswind, popular for many years but no longer in production. The nearest equivalent is probably the Chronomat.

Of course this might be different in the Sherlock universe.

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Thanks for such a detailed explanation. –  Rippy May 20 at 20:15
    
But if he forgot to set his watch back then why did the watch show up the correct time? –  Rippy May 20 at 20:23
    
My guess is that he set back the time after his travel, because this is something you know you have to do. He just did not realize that he also had to correct the date. –  atticae May 20 at 20:27
    
Or he just never changed it during the trip. So it would still be set for London time with just the date being off. –  Roger May 20 at 20:42
1  
If he never changed it at all, it would show the correct date though. The watch does not really care about travel or datelines, and time went on normally for the watch and London (relativity effects omitted), so without any change on the watch the date and time would be correct after the travel. He changed the time without changing the date, that's what lead to the problem. –  atticae May 20 at 20:47

I agree with the answer @atticae gave. But for anyone who is still confused, here is a sample set of steps that would lead to the watch showing the correct time, but being off by 2 days.

Assume today is Wednesday. And all the travel below happens today (which is impossible, but it's simpler for this explanation).

  • You get on a plane at 12:00 noon.
  • When you arrive at your destination, the local time is 5 AM, so you set your watch back 7 hours.
  • You get on another plane.
  • When you arrive at your next destination, the local time is 11 PM, so you set your watch back 6 hours AND THE DAY ON YOUR WATCH IS NOW TUESDAY. That's because you adjusted the time backwards, from 5 AM back to 11 PM.
  • You get on another plane.
  • When you arrive at your destination, the local time is 12 noon. (It's in the same time zone as the airport you originally left from.) You set your watch back to show 12 noon. It still shows Tuesday.
  • You get on another plane.
  • When you arrive at your destination, the local time is 5 AM. You set your watch back. (The date still shows Tuesday.)
  • You get on another plane.
  • When you arrive at your next destination, the local time is 11 PM. You set your watch back another 6 hours AND THE DAY ON YOUR WATCH IS NOW MONDAY.
  • You get on your final plane.
  • It arrives at the first airport. Local time is 12 noon. You set your watch back, and it now shows the correct local time, but the date shows MONDAY.

It's way more complicated than this, because you also have to consider the amount of time you'd spend flying. But the effect is exactly the same: if you keep setting your watch back (instead of forwards) each time you land, and you go around the world twice, your watch will show the correct local time but 2 days in the past.

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A bit strange how it is monday after all this flying. ;) But yes, that's basically how it works. I considered adding an example to my answer, but tbh it was already such a long answer that I didn't think expanding it further would make sense. Also as you noted, it gets quite complex. Good job writing it down, though. –  atticae May 20 at 23:05
    
Thanks man! Your explanation explained it to me. I just laid it out with a specific example. (Now I wish I'd used 8-hour increments for each time zone change....) –  BrettFromLA May 21 at 3:48
    
Thanks for your answer @BrettFromLA. Though seems a bit vague but I guess this is the only logical explanation. –  Rippy May 21 at 7:46

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