AnalogClock: Add feature to change and show time zone

The said time zone is local to each instance of the AnalogClock and is
separate from the system time zone.

This allows user to have clocks that use different time zones
simultaneously.

Userland/Applications/AnalogClock/AnalogClock.cpp

@@ -10,6 +10,7 @@
 #include <LibGUI/Window.h>
 #include <LibGfx/Palette.h>
 #include <LibGfx/Path.h>
+#include <LibTimeZone/TimeZone.h>
 
 void AnalogClock::draw_graduations(GUI::Painter& painter, Gfx::IntRect rect, int x, int y)
 {
@@ -112,9 +113,21 @@ void AnalogClock::paint_event(GUI::PaintEvent& event)
     GUI::Painter painter(*this);
     painter.clear_rect(event.rect(), m_show_window_frame ? palette().window() : Gfx::Color::Transparent);
 
+    if (m_show_time_zone) {
+        painter.draw_text(
+            Gfx::IntRect { { event.rect().width() / 2, (int)(event.rect().height() - m_clock_face_radius) / 2 }, {} },
+            m_time_zone,
+            Gfx::FontDatabase::default_font().bold_variant(),
+            Gfx::TextAlignment::Center);
+    }
+
     draw_face(painter);
 
-    auto time = Core::DateTime::now();
+    auto now_seconds = Core::DateTime::now().timestamp();
+    auto maybe_time_zone_offset = TimeZone::get_time_zone_offset(m_time_zone, UnixDateTime::from_seconds_since_epoch(now_seconds));
+    VERIFY(maybe_time_zone_offset.has_value());
+
+    auto time = Core::DateTime::from_timestamp(now_seconds + maybe_time_zone_offset.value().seconds);
     auto minute = time.minute() * (2 * M_PI) / 60;
     auto hour = (minute + (2 * M_PI) * time.hour()) / 12;
     auto seconds = time.second() * (2 * M_PI) / 60;

Userland/Applications/AnalogClock/AnalogClock.h

@@ -18,6 +18,9 @@ public:
 
     Function<void(GUI::ContextMenuEvent&)> on_context_menu_request;
 
+    void set_time_zone(StringView time_zone) { m_time_zone = time_zone; }
+    void set_show_time_zone(bool value) { m_show_time_zone = value; }
+
 private:
     AnalogClock()
         : m_small_graduation_square(Gfx::IntRect({}, { 3, 3 }))
@@ -38,6 +41,9 @@ private:
 
     bool m_show_window_frame { true };
 
+    StringView m_time_zone;
+    bool m_show_time_zone { false };
+
 protected:
     void context_menu_event(GUI::ContextMenuEvent& event) override;
     void paint_event(GUI::PaintEvent&) override;

Userland/Applications/AnalogClock/main.cpp

@@ -8,12 +8,14 @@
 #include "AnalogClock.h"
 #include <LibCore/DateTime.h>
 #include <LibCore/System.h>
+#include <LibGUI/ActionGroup.h>
 #include <LibGUI/Application.h>
 #include <LibGUI/Icon.h>
 #include <LibGUI/Menu.h>
 #include <LibGUI/Menubar.h>
 #include <LibGUI/Window.h>
 #include <LibMain/Main.h>
+#include <LibTimeZone/TimeZone.h>
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
@@ -40,6 +42,47 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     auto menu = GUI::Menu::construct();
     menu->add_action(*show_window_frame_action);
 
+    menu->add_separator();
+
+    menu->add_action(GUI::Action::create_checkable(
+        "Show Time Zone",
+        [&clock](auto& action) {
+            clock->set_show_time_zone(action.is_checked());
+        }));
+
+    auto timezone_submenu = menu->add_submenu("Time Zone"_string);
+
+    GUI::ActionGroup timezone_action_group;
+    timezone_action_group.set_exclusive(true);
+
+    for (auto time_zone : TimeZone::all_time_zones()) {
+        auto timezone_action = GUI::Action::create_checkable(
+            time_zone.name,
+            [&clock](auto& action) {
+                clock->set_time_zone(action.text());
+            });
+
+        timezone_action_group.add_action(timezone_action);
+        timezone_submenu->add_action(timezone_action);
+    }
+
+    auto reset_to_system_time_zone_action = GUI::Action::create(
+        "Reset to System Time Zone",
+        [&timezone_action_group](auto&) {
+            auto system_time_zone = TimeZone::system_time_zone();
+
+            timezone_action_group.for_each_action([&system_time_zone](auto& action) {
+                if (action.text() == system_time_zone) {
+                    action.activate();
+                    return IterationDecision::Break;
+                }
+                return IterationDecision::Continue;
+            });
+        });
+
+    menu->add_action(reset_to_system_time_zone_action);
+    reset_to_system_time_zone_action->activate();
+
     clock->on_context_menu_request = [&](auto& event) {
         menu->popup(event.screen_position());
     };