Refactors date handling to use DateService

Standardizes date manipulation across the application by leveraging the DateService.
This change improves consistency and reduces code duplication by removing redundant date calculations.

src/utils/DateService.ts

@@ -20,6 +20,7 @@ import {
   startOfWeek,
   endOfWeek,
   addWeeks,
+  addMonths,
   isSameDay,
   getISOWeek
 } from 'date-fns';
@@ -257,6 +258,16 @@ export class DateService {
   public addWeeks(date: Date, weeks: number): Date {
     return addWeeks(date, weeks);
   }
+
+  /**
+   * Add months to a date
+   * @param date - Base date
+   * @param months - Number of months to add (can be negative)
+   * @returns New date
+   */
+  public addMonths(date: Date, months: number): Date {
+    return addMonths(date, months);
+  }
   
   /**
    * Get ISO week number (1-53)