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I would like to know whether \today gets time off of a time server or the system clock, and when. Does it get the time when compiling? Upon viewing the document created?

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If you are working on a machine you own you could experiment by changing the date locally and see what \today shows. My guess is that TeX uses local data. Then you could answer your own question. – Ethan Bolker Dec 24 '12 at 22:05
Welcome to TeX.SE. I don't think it would make sense for an application to get the time of a server (except of course for the the application that sets the system time). An application should only get the time off your system as that would (or should) already be set up get the time of a server and select the appropriate time zone. Otherwise each application would need to allow for one to specify the time zone and which server to use. – Peter Grill Dec 24 '12 at 22:18
TeX was/is written in Pascal and is available "right next to the TeX Book" as tex.web. Even though it was last updated in February 2008 as Version 3.1415926, I doubt whether it was written or intended to extract content from an online source at every compilation. – Werner Dec 24 '12 at 22:45
It pings the international space station. – Nicholas Hamilton Jun 21 '13 at 3:59
up vote 9 down vote accepted

Surely, it gets the system clock, since if it wants to get time of a time server, you have to be online all the time; but as you know, you can easily compile your code, even if you are offline and not connected to internet.

Question #2: when you compile a document, the output will be created and you can see that, whenever you want. While seeing an output pdf, no change will be happened for that pdf file.

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