Getting the Timezone in Regional setting using javascript

With REST:

_api/Web/RegionalSettings/TimeZone

With JSOM:

var ctx = SP.ClientContext.get_current();
var timeZone = ctx.get_web().get_regionalSettings();
ctx.load(timeZone);
ctx.executeQueryAsync(function() {
  console.log(timeZone);
});

2010:
It does not seem easy to do this in 2010. Here is a solution by Marc, which is based of this answer by Waqas Sarwar.
Another options is to use this solution by lem.mallari which use Utility.FormatDateTime to convert a given time to the site timezone. I have converted the last answer from CSOM to JSOM:

var ctx = SP.ClientContext.get_current();
var dt = new Date();
var res = SP.Utilities.Utility.formatDateTime(ctx, ctx.get_web(), dt.toISOString(), SP.Utilities.DateTimeFormat.dateTime);
ctx.executeQueryAsync(function()  {
  console.log(dt, res.get_value());
});

JSOM or JavaScript Object Model is a coding practice in which we use JavaScript API reference for SharePoint 2013. Refer this link to get an idea

REST or Representational State Transfer is introduced in SP2013 that is comparable to the existing SharePoint client object models. Refer this msdn link to get more idea and also this MSDN Magazine

To get the time zone below JavaScript function can be helpful

// Global variable to store time zone object.
var siteTimeZone = '';

// function to get time zone
function getTimeZome(){
        var context = SP.ClientContext.get_current();
        var web = context.get_web();
        var timeZone = web.get_regionalSettings().get_timeZone();
        context.load(timeZone);
        context.executeQueryAsync(
        function onSucceeded() {
            siteTimeZone = timeZone;
        },
         queryFailure
        );

}

// function to handle failuer 
function queryFailure (sender, args) {
    alert('error');
}

//This is the way you call the function, making sure that sp.js is loaded which contains necessary functions.
ExecuteOrDelayUntilScriptLoaded(getTimeZome, "sp.js");