fix: add America/St_Johns handling to tzOffset

Author: allohamora

src/tzOffset/index.ts

@@ -25,8 +25,14 @@ export function tzOffset(timeZone: string | undefined, date: Date): number {
     const offsetStr = format(date).slice(6);
     if (offsetStr in offsetCache) return offsetCache[offsetStr]!;
 
-    const [hours, minutes] = offsetStr.split(":").map(Number);
-    return (offsetCache[offsetStr] = hours! * 60 + (minutes || 0));
+    const [hours, minutes = 0] = offsetStr.split(":").map(Number);
+    
+    // type-guard
+    if (hours === undefined) {
+      return NaN;
+    }
+
+    return (offsetCache[offsetStr] = hours > 0 ? hours * 60 + minutes : hours * 60 - minutes);
   } catch {
     return NaN;
   }

src/tzOffset/tests.ts

@@ -39,6 +39,16 @@ describe("tzOffset", () => {
     expect(tzOffset("Europe/London", date)).toBe(0);
   });
 
+  it('works with America/St_Johns for Daylight Saving Time', () => {
+    const date = new Date("2023-03-15T18:00:00.000Z");
+    expect(tzOffset("America/St_Johns", date)).toBe(-150);
+  });
+
+  it('works America/St_Johns for Standard Time', () => {
+    const date = new Date("2023-03-03T18:00:00.000Z");
+    expect(tzOffset("America/St_Johns", date)).toBe(-210);
+  });
+
   it("returns NaN if the offset the date or time zone are invalid", () => {
     expect(tzOffset("Etc/Invalid", new Date("2020-01-15T00:00:00Z"))).toBe(NaN);
     expect(tzOffset("America/New_York", new Date(NaN))).toBe(NaN);