It can be done using an Ajax request:
UniSession.AddJS(
'var d = new Date(); ' +
'var _offset = (d.getTimezoneOffset() / 60 ) * -1; ' +
'ajaxRequest(MainForm.form, "mycall", ["ofs="+_offset]); '
);
procedure TMainForm.UniFormAjaxEvent(Sender: TComponent; EventName: string;
Params: TStrings);
begin
if EventName='mycall' then
begin
UniLabel1.Caption:=Params.Values['ofs'];
end;
end;