Improves date handling and event stacking

Enhances date validation and timezone handling using DateService, ensuring data integrity and consistency.

Refactors event rendering and dragging to correctly handle date transformations.

Adds a test plan for event stacking and z-index management.

Fixes edge cases in navigation and date calculations for week/year boundaries and DST transitions.

src/utils/DateService.ts

@@ -101,6 +101,16 @@ export class DateService {
   public formatDate(date: Date): string {
     return format(date, 'yyyy-MM-dd');
   }
+
+  /**
+   * Format date as "Month Year" (e.g., "January 2025")
+   * @param date - Date to format
+   * @param locale - Locale for month name (default: 'en-US')
+   * @returns Formatted month and year
+   */
+  public formatMonthYear(date: Date, locale: string = 'en-US'): string {
+    return date.toLocaleDateString(locale, { month: 'long', year: 'numeric' });
+  }
   
   /**
    * Format date as ISO string (same as formatDate for compatibility)
@@ -413,6 +423,76 @@ export class DateService {
   public isValid(date: Date): boolean {
     return isValid(date);
   }
+
+  /**
+   * Validate date range (start must be before or equal to end)
+   * @param start - Start date
+   * @param end - End date
+   * @returns True if valid range
+   */
+  public isValidRange(start: Date, end: Date): boolean {
+    if (!this.isValid(start) || !this.isValid(end)) {
+      return false;
+    }
+    return start.getTime() <= end.getTime();
+  }
+
+  /**
+   * Check if date is within reasonable bounds (1900-2100)
+   * @param date - Date to check
+   * @returns True if within bounds
+   */
+  public isWithinBounds(date: Date): boolean {
+    if (!this.isValid(date)) {
+      return false;
+    }
+    const year = date.getFullYear();
+    return year >= 1900 && year <= 2100;
+  }
+
+  /**
+   * Validate date with comprehensive checks
+   * @param date - Date to validate
+   * @param options - Validation options
+   * @returns Validation result with error message
+   */
+  public validateDate(
+    date: Date,
+    options: {
+      requireFuture?: boolean;
+      requirePast?: boolean;
+      minDate?: Date;
+      maxDate?: Date;
+    } = {}
+  ): { valid: boolean; error?: string } {
+    if (!this.isValid(date)) {
+      return { valid: false, error: 'Invalid date' };
+    }
+
+    if (!this.isWithinBounds(date)) {
+      return { valid: false, error: 'Date out of bounds (1900-2100)' };
+    }
+
+    const now = new Date();
+
+    if (options.requireFuture && date <= now) {
+      return { valid: false, error: 'Date must be in the future' };
+    }
+
+    if (options.requirePast && date >= now) {
+      return { valid: false, error: 'Date must be in the past' };
+    }
+
+    if (options.minDate && date < options.minDate) {
+      return { valid: false, error: `Date must be after ${this.formatDate(options.minDate)}` };
+    }
+
+    if (options.maxDate && date > options.maxDate) {
+      return { valid: false, error: `Date must be before ${this.formatDate(options.maxDate)}` };
+    }
+
+    return { valid: true };
+  }
   
   /**
    * Check if event spans multiple days