// source at https://stackoverflow.com/a/8808066

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You can use Date.UTC(year,month,day,hours,minutes,seconds,msec)

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It operates just like the Date constructor, but returns the timestamp of the arguments at Greenwich time (offset=0) instead of local time.

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var localtime=new Date(Date.UTC(year,month,day,hours,minutes,seconds,msec)) 

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returns the local time for the UTC time specified.

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Everyone (whose clock is set correctly) will end the countdown together.