Seiko 7B24 Manual page 29

There are time differences around the world based on "Coordinated Universal
Time (UTC)." There are 24 regions (Time Zones) around the world with a
time difference of one hour. This system has been adopted internationally by
settingthetotaltimedifferencearoundtheworldas24hours.
In addition, Daylight Saving Time (Summer Time) is individually adopted in
someregions.
[WhatisUTC(CoordinatedUniversalTime)?]
  U TC is the universal standard time coordinated through international agreement.  
It is used as the official time around the world.  UTC is determined by adding
a leap second to GMT (Greenwich Mean Time), which is determined through
astronomicalmeasurement,inordertokeeptheprecisetime.
[Whatissummertime(DST)?]
  S ummer time is daylight saving time.  Advancing the watch one hour to prolong
daytime during longer daylight hours in summer.  Daylight saving time has
been adopted in about 80 countries, mainly in Europe and North America.  The
adoptionanddurationofdaylightsavingtimevarydependingonthecountry.
※ Timedifferencesbetweenregionsanddaylightsavingtimemaychangeduetocircumstancesofacountryorregion.
※ Atimedifferenceestablishedin15minuteor30minuteunitsisadoptedinverylimitedregions(ex.India).
109