When I need to quickly check what time it is in Kuala Lumpur right now and what time it will be there tomorrow after lunch CET, I automatically type the address into the browser tmzns.com and a great calculator for normal people by Filip Hřáček will calculate it in a few milliseconds.


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Timezones for humans converts the common coordinated world time (UTC) to the time zone of the desired destination, where, as we all learned long ago in school, each of these zones differs from each other by one hour, and ideally its axis is a multiple of the 15th meridian.

When it is 18:38 on Sněžka, it is only 18:23 in Aš

Mapy.cz recently released new ones REST API for geo app developers, so let’s try it out today and make one a slightly different calculator. A calculator Timezones for nerdswhich will not calculate what the time zone is in the given place, but the local one – meridian.

That is, the kind of time that really corresponds to the earth’s rotation with a resolution not of whole hourly leaps, but of individual seconds, and so even tourists on the Charles Bridge and a few kilometers away at the Prague airport already have it.

Timezones for nerds

One Earth rotation (360°) takes 24 hours. Two places on Earth that are 1° longitude apart are therefore 240 seconds apart different local timeto which all surrounding natural phenomena, the actual times of sunrise and sunset, and so on, correspond at the same latitude.

The calculator calculates this time difference for any two places in the Czech Republic. Just fill in the name of a point, city or address, API from Seznam will try to find its location, and our program will take care of the rest:

The calculator calculates from the location of a pair of places in the Czech Republic their local (meridian) time relative to the coordinated world time UTC and precisely the time shift. You might be surprised how enormous differences arise even in our relatively small territory.

Click for larger image
Even in the small Czech Republic, we have huge differences in local times

Therefore, the Central European zone time best intersects with the local meridian only in the immediate vicinity of 15° longitude. For example in Jindřichov Hradec. But as soon as we move east and west from Jindřichov Hradec, the difference starts to grow with each kilometer.

First, let’s explain what our calculator actually calculated. We will show what the new REST API from Seznam can actually do, and at the end, as always, you will find the complete and commented source code of the calculator in HTML and Javascript on a tray. Completely identical to the one that probably started correctly in your web browser in the block above.

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What is the difference between the time zone on a watch and the local time derived from the earth’s rotation.

Mapy.cz REST API is free for small projects and DIYers. We’ll show you everything you need.

At the end, you will find the complete and commented source code of the calculator in Javascript and HTML. You can also run it in your browser.

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