diff --git a/src/managers/GridManager.ts b/src/managers/GridManager.ts
index 6aceac4..358e09d 100644
--- a/src/managers/GridManager.ts
+++ b/src/managers/GridManager.ts
@@ -5,6 +5,7 @@ import { calendarConfig } from '../core/CalendarConfig';
 import { EventTypes } from '../constants/EventTypes';
 import { StateEvents } from '../types/CalendarState';
 import { DateUtils } from '../utils/DateUtils';
+import { DateCalculator } from '../utils/DateCalculator';
 import { ResourceCalendarData } from '../types/CalendarTypes';
 import { GridRenderer } from '../renderers/GridRenderer';
 import { GridStyleManager } from '../renderers/GridStyleManager';
@@ -29,11 +30,13 @@ export class GridManager {
   private resourceData: ResourceCalendarData | null = null; // Store resource data for resource calendar
   private gridRenderer: GridRenderer;
   private styleManager: GridStyleManager;
+  private dateCalculator: DateCalculator;
   
   constructor() {
     console.log('🏗️ GridManager: Constructor called');
     this.gridRenderer = new GridRenderer(calendarConfig);
     this.styleManager = new GridStyleManager(calendarConfig);
+    this.dateCalculator = new DateCalculator(calendarConfig);
     this.init();
   }
 
@@ -43,7 +46,7 @@ export class GridManager {
     
     // Set initial current week to today if not set
     if (!this.currentWeek) {
-      this.currentWeek = this.getWeekStart(new Date());
+      this.currentWeek = this.dateCalculator.getISOWeekStart(new Date());
       console.log('GridManager: Set initial currentWeek to', this.currentWeek);
       // Don't render immediately - wait for proper initialization event
       console.log('GridManager: Waiting for initialization complete before rendering');
@@ -51,8 +54,8 @@ export class GridManager {
   }
 
   private getWeekStart(date: Date): Date {
-    // Use DateUtils for consistent week calculation (Sunday = 0)
-    return DateUtils.getWeekStart(date, 0);
+    // Use DateCalculator for ISO week (Monday = start)
+    return this.dateCalculator.getISOWeekStart(date);
   }
 
   private findElements(): void {
@@ -84,14 +87,21 @@ export class GridManager {
     // Re-render on period change
     eventBus.on(EventTypes.PERIOD_CHANGE, (e: Event) => {
       const detail = (e as CustomEvent).detail;
-      this.currentWeek = detail.week;
+      this.currentWeek = this.dateCalculator.getISOWeekStart(detail.week);
       this.render();
     });
 
     // Handle week changes from NavigationManager
     eventBus.on(EventTypes.WEEK_CHANGED, (e: Event) => {
       const detail = (e as CustomEvent).detail;
-      this.currentWeek = detail.weekStart;
+      this.currentWeek = this.dateCalculator.getISOWeekStart(detail.weekStart);
+      this.render();
+    });
+
+    // Handle date changes from CalendarManager
+    eventBus.on(EventTypes.DATE_CHANGED, (e: Event) => {
+      const detail = (e as CustomEvent).detail;
+      this.currentWeek = this.dateCalculator.getISOWeekStart(detail.currentDate);
       this.render();
     });
 
diff --git a/src/utils/DateCalculator.ts b/src/utils/DateCalculator.ts
index 3ca32f5..be1574d 100644
--- a/src/utils/DateCalculator.ts
+++ b/src/utils/DateCalculator.ts
@@ -13,29 +13,27 @@ export class DateCalculator {
   }
 
   /**
-   * Get dates for work week based on week start and work days configuration
-   * @param weekStart - The start date of the week (could be Sunday or Monday)
+   * Get dates for work week based on ISO week (starts Monday)
+   * @param weekStart - Any date in the week
    * @returns Array of dates for the configured work days
    */
   getWorkWeekDates(weekStart: Date): Date[] {
     const dates: Date[] = [];
     const workWeekSettings = this.config.getWorkWeekSettings();
-    const dateSettings = this.config.getDateViewSettings();
-    const firstDayOfWeek = dateSettings.firstDayOfWeek; // 0=Sunday, 1=Monday
     
-    // Get the actual start of the week based on configuration
-    const actualWeekStart = this.getWeekStart(weekStart, firstDayOfWeek);
+    // Get Monday of the ISO week
+    const monday = this.getISOWeekStart(weekStart);
     
     // Calculate dates for each work day
     workWeekSettings.workDays.forEach(dayOfWeek => {
-      const date = new Date(actualWeekStart);
+      const date = new Date(monday);
       
-      if (firstDayOfWeek === 1 && dayOfWeek === 0) {
-        // For Monday-based weeks, Sunday is at the end (day 7)
-        date.setDate(actualWeekStart.getDate() + 7);
+      if (dayOfWeek === 0) {
+        // Sunday is 6 days after Monday
+        date.setDate(monday.getDate() + 6);
       } else {
-        // Normal calculation
-        date.setDate(actualWeekStart.getDate() + dayOfWeek);
+        // Monday=1 becomes 0 days after, Tuesday=2 becomes 1 day after, etc.
+        date.setDate(monday.getDate() + dayOfWeek - 1);
       }
       
       dates.push(date);
@@ -45,26 +43,27 @@ export class DateCalculator {
   }
 
   /**
-   * Get the start of the week for a given date
+   * Get the start of the ISO week (Monday) for a given date
+   * @param date - Any date in the week
+   * @returns The Monday of the ISO week
+   */
+  getISOWeekStart(date: Date): Date {
+    const monday = new Date(date);
+    const currentDay = monday.getDay();
+    const daysToSubtract = currentDay === 0 ? 6 : currentDay - 1;
+    monday.setDate(monday.getDate() - daysToSubtract);
+    monday.setHours(0, 0, 0, 0);
+    return monday;
+  }
+
+  /**
+   * Get the start of the week for a given date (legacy method)
    * @param date - Any date in the week
    * @param firstDayOfWeek - 0 for Sunday, 1 for Monday
    * @returns The start date of the week
    */
   getWeekStart(date: Date, firstDayOfWeek: number = 1): Date {
-    const weekStart = new Date(date);
-    const currentDay = weekStart.getDay();
-    
-    if (firstDayOfWeek === 1) {
-      // Monday as first day
-      const daysToSubtract = currentDay === 0 ? 6 : currentDay - 1;
-      weekStart.setDate(weekStart.getDate() - daysToSubtract);
-    } else {
-      // Sunday as first day
-      weekStart.setDate(weekStart.getDate() - currentDay);
-    }
-    
-    weekStart.setHours(0, 0, 0, 0);
-    return weekStart;
+    return this.getISOWeekStart(date);
   }
 
   /**