# Time Zones at Sea? **URL:** \ **Category:** Factual Questions\ **Created:** [March 31, 2009, 9:44am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405 "2009-03-31T09:44:19Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![WormTheRed](https://avatars.discourse-cdn.com/v4/letter/w/bbe5ce/32.png) [@WormTheRed](https://boards.straightdope.com/u/WormTheRed)\ **Post date:** [March 31, 2009, 9:44am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/1 "2009-03-31T09:44:19Z")
I was flying back to Iceland yesterday and since they didn’t want to give me any headphones and normal ones don’t work I was pretty much stuck with playing with the flight-information on the touch screen in front of me. As I was doing that I noticed that the screen suddenly jumped an hour when we were up in the air, supposedly when we passed a time zone line and that got me thinking about the following question: Do sailors on those big ocean going vessels change their clocks when they pass into a new time zone? Let’s for example think of a big container ship that goes from Japan to Germany; would they always go on local time? ETA: I seem to remember that the US Navy always acts on Zulu Time, so if that’s true, my question is mostly about civilian vessels. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [March 31, 2009, 10:20am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/2 "2009-03-31T10:20:36Z")
AFAIK, in the open ocean (outside territorial waters), the time zones are defined in 15-degree slices of longitude, with each hour centred on the relevant meridian, so, for instance, the GMT zone extends from 7.5ºW to 7.5ºE, the GMT+1 zone from 7.5ºE to 22.5ºE, and so on. [This site](http://home.tiscali.nl/~t876506/Multizones.html) supports that: > [@](#): > > Ships at sea in international waters should set their clocks according to the natural zone time. This is defined as follows. A ‘zone’ runs from -7.5 degrees to +7.5 degrees around a zone meridian. A zone meridian is an integer number of 15 degrees. For each zone easterly from the 0-zone 1 hour is added to the UTC time, and for each zone westerly 1 hour is subtracted. Within a zone the time offset is constant. Clocks are changed as is convenient, usually on a whole hour, when crossing a natural zone time boundary. As soon as the territorial waters of a nation is reached, the clock is set to the local time of that (part of the) nation. The international date line has no effect; only the 180° meridian. I’m not quite sure what that last part means, as I thought the International Date Line was the 180º meridian, except in territorial waters where it follows the boundaries, keeping nations all in one day. ---
**Author:** ![Harvey\_The\_Heavy](https://avatars.discourse-cdn.com/v4/letter/h/c5a1d2/32.png) [@Harvey\_The\_Heavy](https://boards.straightdope.com/u/Harvey_The_Heavy)\ **Post date:** [March 31, 2009, 10:44am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/3 "2009-03-31T10:44:45Z")
U.S. Navy ships change ship’s time as they cross each zone. IIRC, the changes were announced the day before. ---
**Author:** ![WormTheRed](https://avatars.discourse-cdn.com/v4/letter/w/bbe5ce/32.png) [@WormTheRed](https://boards.straightdope.com/u/WormTheRed)\ **Post date:** [March 31, 2009, 10:46am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/4 "2009-03-31T10:46:00Z")
Hmm… then it would be pretty darn neat to time your “shifts” to correspond with time zone changes, so you always get an hour less of work, if you’re going to the east. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [March 31, 2009, 11:19am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/5 "2009-03-31T11:19:57Z")
On merchant ships normally the time changes on the 12-4 AM watch. It is posted the day before. I do not know if it is done before crossing or after, ya would have to ask a deckie. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [March 31, 2009, 11:59am UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/6 "2009-03-31T11:59:42Z")
Back in the mid 20th century, I made several long sea voyages between Australia and Europe. (It was a time when air travel was relatively more expensive than it is now, so most long journeys were by sea). As several have said, on some days they would announce that the clocks would change on the next day, and I assume they changed some time shortly after midnight. (I wsas too young to stay up that late). I also assume that they tried to keep up roughly with the 15 degree zones, but changing the clocks in the middle of the night was more important than being exactly right with those zones. You’d also want to have the time the same as local time when you arrived at your next port – so you might need to change the clocks by 30 minutes if the next port was Adelaide or Bombay. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [March 31, 2009, 12:01pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/7 "2009-03-31T12:01:14Z")
> [@WormTheRed](#): > > I seem to remember that the US Navy always acts on Zulu Time… Things would get a bit strange with the daily schedule if this were the case - breakfast at sunset, for example. Zulu time is used for certain things (like recording the arrival of messages) but, as HtH notes, local time zones are observed. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [March 31, 2009, 1:55pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/8 "2009-03-31T13:55:47Z")
> [@Xema](#): > > Things would get a bit strange with the daily schedule if this were the case - breakfast at sunset, for example. Zulu time is used for certain things (like recording the arrival of messages) but, as HtH notes, local time zones are observed. U.S. submarines at sea often switch to Zulu time (aka GMT). Before going up to periscope depth, you had to check what the local time was to see if it was night or day on the surface. ---
**Author:** ![Kent\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kent_clark/32/105_2.png) [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)\ **Post date:** [March 31, 2009, 2:18pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/9 "2009-03-31T14:18:01Z")
On my one and only transatlantic trip, the ship reset the clocks each night. It did mean we ate our meals an hour earlier each day. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [March 31, 2009, 2:18pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/10 "2009-03-31T14:18:53Z")
> [@Colophon](#): > > I’m not quite sure what that last part means, as I thought the International Date Line was the 180º meridian, except in territorial waters where it follows the boundaries, keeping nations all in one day. Parts of the deviations from 180º occur well outside of territorial waters - for example south-east of New Zealand: [http://upload.wikimedia.org/wikipedia/commons/6/61/International\_Date\_Line.png](http://upload.wikimedia.org/wikipedia/commons/6/61/International_Date_Line.png) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [March 31, 2009, 2:26pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/11 "2009-03-31T14:26:21Z")
On a nine day trip to Europe, the ship made an hour change every night for the first five. I had thought it would help me adjust to the time change more readily, but I found five one hour changes harder than a single five hour change. I don’t think they paid any attention to the actual time zones. ---
**Author:** ![WormTheRed](https://avatars.discourse-cdn.com/v4/letter/w/bbe5ce/32.png) [@WormTheRed](https://boards.straightdope.com/u/WormTheRed)\ **Post date:** [March 31, 2009, 2:54pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/12 "2009-03-31T14:54:55Z")
> [@Xema](#): > > Things would get a bit strange with the daily schedule if this were the case - breakfast at sunset, for example. Zulu time is used for certain things (like recording the arrival of messages) but, as HtH notes, local time zones are observed. True. Didn’t think of that :smack: And thank you all for the input! My ignorance has been fought. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [March 31, 2009, 2:58pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/13 "2009-03-31T14:58:30Z")
> [@robby](#): > > U.S. submarines at sea often switch to Zulu time (aka GMT). Makes sense if you’re going to be submerged for months, with no pesky sunlight to disrupt things. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [March 31, 2009, 3:00pm UTC](https://boards.straightdope.com/t/time-zones-at-sea/491405/14 "2009-03-31T15:00:48Z")
> [@WormTheRed](#): > > Hmm… then it would be pretty darn neat to time your “shifts” to correspond with time zone changes, so you always get an hour less of work, if you’re going to the east. It also means an hour more of work each day when the ship is traveling in the other direction.