Question

How is standard time useful?

Answer 1

there are too many longitudes passing though a single country, if there is no standard time then it'll confuse people when they travel from one parcel to another in their own country.

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Answer 2

Standard time is useful as if in large countries we follow different time for different regions , it will become very hard for us to maintain jobs and services which are done at national level and keeping record wouldn't be easy, So its more convenient to follow a standard time rather than having different time for different regions and the different between local time and standard time isn't much.