How do Social Networking applications convert server time according to viewer's timezone

How to convert the UTC time according to viewer's timezone?

I heard that it's not possible to get the timezone of the viewer, but then ho开发者_运维技巧w do social networking services like FB, twitter do this ?

Edit: I have registered users on my application.

• It is possible to get the user timezone, but this is done with javascript (get the detect_timezone.js). You most likely do that when users are registered.

• Another, worse way is to approximate the timezone based on the IP

• and finally (with combination of any of the above), you can give the user a choice to select his timezone in his settings