Use macOS API to retrieve the local timezone

The `time` crate currently doesn't have a reliable way to get that.
In the future, `NSSystemTimeZoneDidChangeNotification` could be
used to keep the cached timezone up-to-date.

Co-Authored-By: Max Brunsfeld <max@zed.dev>

gpui/src/platform/mac/platform.rs

@@ -42,6 +42,7 @@ use std::{
     slice, str,
     sync::Arc,
 };
+use time::UtcOffset;
 
 const MAC_PLATFORM_IVAR: &'static str = "platform";
 static mut APP_CLASS: *const Class = ptr::null();
@@ -558,6 +559,14 @@ impl platform::Platform for MacPlatform {
             let _: () = msg_send![cursor, set];
         }
     }
+
+    fn local_timezone(&self) -> UtcOffset {
+        unsafe {
+            let local_timezone: id = msg_send![class!(NSTimeZone), localTimeZone];
+            let seconds_from_gmt: NSInteger = msg_send![local_timezone, secondsFromGMT];
+            UtcOffset::from_whole_seconds(seconds_from_gmt.try_into().unwrap()).unwrap()
+        }
+    }
 }
 
 unsafe fn get_foreground_platform(object: &mut Object) -> &MacForegroundPlatform {

gpui/src/platform/test.rs

@@ -9,6 +9,7 @@ use std::{
     rc::Rc,
     sync::Arc,
 };
+use time::UtcOffset;
 
 pub struct Platform {
     dispatcher: Arc<dyn super::Dispatcher>,
@@ -136,6 +137,10 @@ impl super::Platform for Platform {
     fn set_cursor_style(&self, style: CursorStyle) {
         *self.cursor.lock() = style;
     }
+
+    fn local_timezone(&self) -> UtcOffset {
+        UtcOffset::UTC
+    }
 }
 
 impl Window {