All counties in Michigan's Lower Peninsula observe Eastern Time, as do most counties in the state's Upper Peninsula. The U.P. counties that do not, the four counties which have a land border with Wisconsin, are in the Central Time zone.
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History
Before time zones were introduced, every place used local observation of the sun to set their clocks, which means they used local mean time, every city different based on their longitude. Detroit used 05:32:11 west of Greenwich and Menominee 05:51:31 west of Greenwich.
Time zones were introduced in the United States in 1883. This was in introduced in different years based on local decisions. Detroit introduced Central time on January 1, 1905. and Menominee on 18 September 18, 1885. Later Detroit and most of Michigan changed to Eastern time.
In 1967, when the Uniform Time Act came into effect, the Upper Peninsula went under year-round CST, with no daylight saving time. In 1973, the majority of the peninsula switched to Eastern Time; only the four western counties of Gogebic, Iron, Dickinson, and Menominee continue to observe Central Time.
IANA time zone database
The zone for Michigan as given by zone.tab of the IANA time zone database