How do they know (or detect) what time it is?

Question

I am reading a book about javascript and there is sample code there which greets you and message differently according to what time it is. I am just curious how it works. Does the javascript use my PC's time or on internet's?

Answer 1

In simple ans : The browser uses your PC's time.

If you wanna know more google Unix Time Stamp.
Don feel like explaining. Its an interesting time.