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I read somewhere, that if you set a timer before Shabbat, you are not allowed to adjust the timer or clock on Shabbat to reset the time that something goes on.

So, let's say that you live in Eastville in Eastern time zone. You visit your friend who lives < 2,000 amot from you in Centville in Central Time zone (1 hour behind), on Shabbat. There is an eruv between the two cities.

You have a battery operated watch with an alarm that has a built-in GPS clock so the GPS automatically changes the both the clock time as well as the alarm setting based on where you are.

Shabbat starts at 6 P.M. Eastern time, (5 P.M. central time.) You set the alarm to go on at 7 P.M. - 2 hours from now. You begin walking to your friend at 6:15, and cross the time zone at 6:30, when the GPS resets the clock time to 5:30 and also recalculates the time for the alarm at 6 P.M. (otherwise, you'd have to wait an extra hour.)

Are you allowed to carry this type of watch to your friend on Shabbat? Granted that the time adjustments are automatically done, but is there a problem of your causing this auto change to occur?

I used the alarm watch as an example. The answer can apply to any similar device.

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    What about a lightbulb attached to a GPS that turns on when you bring it from your living room to your bedroom? – Double AA May 19 '15 at 20:25
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    Can you carry it at all? – Scimonster May 20 '15 at 11:18
  • @DoubleAA or if it operates as a remote control to turn on your oven? Do we have to venture into KOSHER Switch territory... – Isaac Kotlicky May 20 '15 at 11:26
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    What about carrying one of those keys that's location sensitive near a car? You're not actually ever putting it in the ignition! – Double AA May 20 '15 at 14:07

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