Optimize performance: cache dates, reduce lookups, improve algorithms

Co-authored-by: THoguet <61246905+THoguet@users.noreply.github.com>

src/app/services/calendar-service.spec.ts

@@ -1,13 +1,13 @@
 import { TestBed } from '@angular/core/testing';
 
-import { Calendar } from './calendar';
+import { CalendarService } from './calendar-service';
 
-describe('Calendar', () => {
-	let service: Calendar;
+describe('CalendarService', () => {
+	let service: CalendarService;
 
 	beforeEach(() => {
 		TestBed.configureTestingModule({});
-		service = TestBed.inject(Calendar);
+		service = TestBed.inject(CalendarService);
 	});
 
 	it('should be created', () => {