diff --git a/src/managers/CalendarManager.ts b/src/managers/CalendarManager.ts
index 3fda062..2cf9e20 100644
--- a/src/managers/CalendarManager.ts
+++ b/src/managers/CalendarManager.ts
@@ -6,6 +6,7 @@ import { EventManager } from './EventManager.js';
 import { GridManager } from './GridManager.js';
 import { EventRenderingService } from '../renderers/EventRendererManager.js';
 import { ScrollManager } from './ScrollManager.js';
+import { DateCalculator } from '../utils/DateCalculator.js';
 
 /**
  * CalendarManager - Main coordinator for all calendar managers
@@ -18,6 +19,7 @@ export class CalendarManager {
     private gridManager: GridManager;
     private eventRenderer: EventRenderingService;
     private scrollManager: ScrollManager;
+    private dateCalculator: DateCalculator;
     private currentView: CalendarView = 'week';
     private currentDate: Date = new Date();
     private isInitialized: boolean = false;
@@ -36,6 +38,7 @@ export class CalendarManager {
         this.gridManager = gridManager;
         this.eventRenderer = eventRenderer;
         this.scrollManager = scrollManager;
+        this.dateCalculator = new DateCalculator(config);
         this.setupEventListeners();
         console.log('📋 CalendarManager: Created with proper dependency injection');
     }
@@ -428,8 +431,8 @@ export class CalendarManager {
         // Trigger event rendering for the current date range using correct method
         this.eventRenderer.renderEvents({
             container: container as HTMLElement,
-            startDate: new Date(periodData.startDate), 
-            endDate: new Date(periodData.endDate)
+            startDate: new Date(periodData.start), 
+            endDate: new Date(periodData.end)
         });
     }
 
@@ -471,10 +474,10 @@ export class CalendarManager {
         const lastDate = new Date(lastDateStr);
         
         // Calculate week number from first date
-        const weekNumber = this.getWeekNumber(firstDate);
+        const weekNumber = this.dateCalculator.getWeekNumber(firstDate);
         
         // Format date range
-        const dateRange = this.formatDateRange(firstDate, lastDate);
+        const dateRange = this.dateCalculator.formatDateRange(firstDate, lastDate);
         
         console.log('CalendarManager: Week info from columns:', {
             firstDate: firstDateStr,
@@ -492,41 +495,5 @@ export class CalendarManager {
         });
     }
 
-    /**
-     * Helper method to get week number
-     */
-    private getWeekNumber(date: Date): number {
-        const start = new Date(date.getFullYear(), 0, 1);
-        const days = Math.floor((date.getTime() - start.getTime()) / (24 * 60 * 60 * 1000));
-        return Math.ceil((days + start.getDay() + 1) / 7);
-    }
-
-    /**
-     * Helper method to get week start (Sunday)
-     */
-    private getWeekStart(date: Date): Date {
-        const weekStart = new Date(date);
-        weekStart.setDate(date.getDate() - date.getDay());
-        return weekStart;
-    }
-
-    /**
-     * Helper method to format date range
-     */
-    private formatDateRange(start: Date, end: Date): string {
-        const months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
-        
-        const startMonth = months[start.getMonth()];
-        const endMonth = months[end.getMonth()];
-        const startDay = start.getDate();
-        const endDay = end.getDate();
-        const year = start.getFullYear();
-        
-        if (startMonth === endMonth) {
-            return `${startMonth} ${startDay} - ${endDay}, ${year}`;
-        } else {
-            return `${startMonth} ${startDay} - ${endMonth} ${endDay}, ${year}`;
-        }
-    }
 
 }
\ No newline at end of file
diff --git a/src/renderers/ColumnRenderer.ts b/src/renderers/ColumnRenderer.ts
index a6e55f9..19a896e 100644
--- a/src/renderers/ColumnRenderer.ts
+++ b/src/renderers/ColumnRenderer.ts
@@ -2,6 +2,7 @@
 
 import { CalendarConfig } from '../core/CalendarConfig';
 import { ResourceCalendarData } from '../types/CalendarTypes';
+import { DateCalculator } from '../utils/DateCalculator';
 
 /**
  * Interface for column rendering strategies
@@ -23,10 +24,15 @@ export interface ColumnRenderContext {
  * Date-based column renderer (original functionality)
  */
 export class DateColumnRenderer implements ColumnRenderer {
+  private dateCalculator: DateCalculator;
+
   render(columnContainer: HTMLElement, context: ColumnRenderContext): void {
     const { currentWeek, config } = context;
     
-    const dates = this.getWeekDates(currentWeek, config);
+    // Initialize date calculator
+    this.dateCalculator = new DateCalculator(config);
+    
+    const dates = this.dateCalculator.getWorkWeekDates(currentWeek);
     const dateSettings = config.getDateViewSettings();
     const daysToShow = dates.slice(0, dateSettings.weekDays);
     
@@ -34,7 +40,7 @@ export class DateColumnRenderer implements ColumnRenderer {
 
     daysToShow.forEach((date) => {
       const column = document.createElement('swp-day-column');
-      (column as any).dataset.date = this.formatDate(date);
+      (column as any).dataset.date = this.dateCalculator.formatISODate(date);
       
       const eventsLayer = document.createElement('swp-events-layer');
       column.appendChild(eventsLayer);
@@ -43,26 +49,6 @@ export class DateColumnRenderer implements ColumnRenderer {
     });
   }
 
-  private getWeekDates(weekStart: Date, config: CalendarConfig): Date[] {
-    const dates: Date[] = [];
-    const workWeekSettings = config.getWorkWeekSettings();
-    
-    // Calculate dates based on actual work days (e.g., [1,2,3,4] for Mon-Thu)
-    workWeekSettings.workDays.forEach(dayOfWeek => {
-      const date = new Date(weekStart);
-      // Set to the start of the week (Sunday = 0)
-      date.setDate(weekStart.getDate() - weekStart.getDay());
-      // Add the specific day of week
-      date.setDate(date.getDate() + dayOfWeek);
-      dates.push(date);
-    });
-    
-    return dates;
-  }
-
-  private formatDate(date: Date): string {
-    return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
-  }
 }
 
 /**
diff --git a/src/renderers/HeaderRenderer.ts b/src/renderers/HeaderRenderer.ts
index cf2d734..95488a0 100644
--- a/src/renderers/HeaderRenderer.ts
+++ b/src/renderers/HeaderRenderer.ts
@@ -2,6 +2,7 @@
 
 import { CalendarConfig } from '../core/CalendarConfig';
 import { ResourceCalendarData } from '../types/CalendarTypes';
+import { DateCalculator } from '../utils/DateCalculator';
 
 /**
  * Interface for header rendering strategies
@@ -24,17 +25,22 @@ export interface HeaderRenderContext {
  * Date-based header renderer (original functionality)
  */
 export class DateHeaderRenderer implements HeaderRenderer {
+  private dateCalculator: DateCalculator;
+
   render(calendarHeader: HTMLElement, context: HeaderRenderContext): void {
     const { currentWeek, config, allDayEvents = [] } = context;
     
-    const dates = this.getWeekDates(currentWeek, config);
+    // Initialize date calculator with config
+    this.dateCalculator = new DateCalculator(config);
+    
+    const dates = this.dateCalculator.getWorkWeekDates(currentWeek);
     const weekDays = config.get('weekDays');
     const daysToShow = dates.slice(0, weekDays);
 
     const workWeekSettings = config.getWorkWeekSettings();
     daysToShow.forEach((date, index) => {
       const header = document.createElement('swp-day-header');
-      if (this.isToday(date)) {
+      if (this.dateCalculator.isToday(date)) {
         (header as any).dataset.today = 'true';
       }
       
@@ -42,7 +48,7 @@ export class DateHeaderRenderer implements HeaderRenderer {
         <swp-day-name>${workWeekSettings.dayNames[index]}</swp-day-name>
         <swp-day-date>${date.getDate()}</swp-day-date>
       `;
-      (header as any).dataset.date = this.formatDate(date);
+      (header as any).dataset.date = this.dateCalculator.formatISODate(date);
       
       calendarHeader.appendChild(header);
     });
@@ -54,7 +60,7 @@ export class DateHeaderRenderer implements HeaderRenderer {
   private renderAllDayEvents(calendarHeader: HTMLElement, context: HeaderRenderContext): void {
     const { currentWeek, config, allDayEvents = [] } = context;
     
-    const dates = this.getWeekDates(currentWeek, config);
+    const dates = this.dateCalculator.getWorkWeekDates(currentWeek);
     const weekDays = config.get('weekDays');
     const daysToShow = dates.slice(0, weekDays);
 
@@ -68,8 +74,8 @@ export class DateHeaderRenderer implements HeaderRenderer {
       let endColumnIndex = -1;
       
       daysToShow.forEach((date, index) => {
-        const dateStr = this.formatDate(date);
-        const startDateStr = this.formatDate(startDate);
+        const dateStr = this.dateCalculator.formatISODate(date);
+        const startDateStr = this.dateCalculator.formatISODate(startDate);
         
         if (dateStr === startDateStr) {
           startColumnIndex = index;
@@ -102,31 +108,6 @@ export class DateHeaderRenderer implements HeaderRenderer {
     });
   }
 
-  private getWeekDates(weekStart: Date, config: CalendarConfig): Date[] {
-    const dates: Date[] = [];
-    const workWeekSettings = config.getWorkWeekSettings();
-    
-    // Calculate dates based on actual work days (e.g., [1,2,3,4] for Mon-Thu)
-    workWeekSettings.workDays.forEach(dayOfWeek => {
-      const date = new Date(weekStart);
-      // Set to the start of the week (Sunday = 0)
-      date.setDate(weekStart.getDate() - weekStart.getDay());
-      // Add the specific day of week
-      date.setDate(date.getDate() + dayOfWeek);
-      dates.push(date);
-    });
-    
-    return dates;
-  }
-
-  private isToday(date: Date): boolean {
-    const today = new Date();
-    return date.toDateString() === today.toDateString();
-  }
-
-  private formatDate(date: Date): string {
-    return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
-  }
 
 }
 
diff --git a/src/renderers/NavigationRenderer.ts b/src/renderers/NavigationRenderer.ts
index bc22ce3..0d3f614 100644
--- a/src/renderers/NavigationRenderer.ts
+++ b/src/renderers/NavigationRenderer.ts
@@ -2,6 +2,7 @@ import { IEventBus } from '../types/CalendarTypes';
 import { EventTypes } from '../constants/EventTypes';
 import { DateUtils } from '../utils/DateUtils';
 import { CalendarConfig } from '../core/CalendarConfig';
+import { DateCalculator } from '../utils/DateCalculator';
 
 /**
  * NavigationRenderer - Handles DOM rendering for navigation containers
@@ -10,10 +11,12 @@ import { CalendarConfig } from '../core/CalendarConfig';
 export class NavigationRenderer {
   private eventBus: IEventBus;
   private config: CalendarConfig;
+  private dateCalculator: DateCalculator;
 
   constructor(eventBus: IEventBus, config: CalendarConfig) {
     this.eventBus = eventBus;
     this.config = config;
+    this.dateCalculator = new DateCalculator(config);
     this.setupEventListeners();
   }
 
@@ -99,17 +102,14 @@ export class NavigationRenderer {
     header.innerHTML = '';
     dayColumns.innerHTML = '';
 
-    // Render headers for target week
+    // Get dates using DateCalculator
+    const dates = this.dateCalculator.getWorkWeekDates(weekStart);
     const workWeekSettings = this.config.getWorkWeekSettings();
-    workWeekSettings.workDays.forEach((dayOfWeek, i) => {
-      const date = new Date(weekStart);
-      // Set to the start of the week (Sunday = 0)
-      date.setDate(weekStart.getDate() - weekStart.getDay());
-      // Add the specific day of week
-      date.setDate(date.getDate() + dayOfWeek);
-      
+    
+    // Render headers for target week
+    dates.forEach((date, i) => {
       const headerElement = document.createElement('swp-day-header');
-      if (this.isToday(date)) {
+      if (this.dateCalculator.isToday(date)) {
         headerElement.dataset.today = 'true';
       }
       
@@ -117,20 +117,15 @@ export class NavigationRenderer {
         <swp-day-name>${workWeekSettings.dayNames[i]}</swp-day-name>
         <swp-day-date>${date.getDate()}</swp-day-date>
       `;
-      headerElement.dataset.date = this.formatDate(date);
+      headerElement.dataset.date = this.dateCalculator.formatISODate(date);
       
       header.appendChild(headerElement);
     });
     
     // Render day columns for target week
-    workWeekSettings.workDays.forEach(dayOfWeek => {
+    dates.forEach(date => {
       const column = document.createElement('swp-day-column');
-      const date = new Date(weekStart);
-      // Set to the start of the week (Sunday = 0)
-      date.setDate(weekStart.getDate() - weekStart.getDay());
-      // Add the specific day of week
-      date.setDate(date.getDate() + dayOfWeek);
-      column.dataset.date = this.formatDate(date);
+      column.dataset.date = this.dateCalculator.formatISODate(date);
       
       const eventsLayer = document.createElement('swp-events-layer');
       column.appendChild(eventsLayer);
@@ -139,18 +134,4 @@ export class NavigationRenderer {
     });
   }
 
-  /**
-   * Utility method to format date
-   */
-  private formatDate(date: Date): string {
-    return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
-  }
-
-  /**
-   * Check if date is today
-   */
-  private isToday(date: Date): boolean {
-    const today = new Date();
-    return date.toDateString() === today.toDateString();
-  }
 }
\ No newline at end of file
diff --git a/src/utils/DateCalculator.ts b/src/utils/DateCalculator.ts
new file mode 100644
index 0000000..3ca32f5
--- /dev/null
+++ b/src/utils/DateCalculator.ts
@@ -0,0 +1,194 @@
+/**
+ * DateCalculator - Centralized date calculation logic for calendar
+ * Handles all date computations with proper week start handling
+ */
+
+import { CalendarConfig } from '../core/CalendarConfig';
+
+export class DateCalculator {
+  private config: CalendarConfig;
+  
+  constructor(config: CalendarConfig) {
+    this.config = config;
+  }
+
+  /**
+   * 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)
+   * @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);
+    
+    // Calculate dates for each work day
+    workWeekSettings.workDays.forEach(dayOfWeek => {
+      const date = new Date(actualWeekStart);
+      
+      if (firstDayOfWeek === 1 && dayOfWeek === 0) {
+        // For Monday-based weeks, Sunday is at the end (day 7)
+        date.setDate(actualWeekStart.getDate() + 7);
+      } else {
+        // Normal calculation
+        date.setDate(actualWeekStart.getDate() + dayOfWeek);
+      }
+      
+      dates.push(date);
+    });
+    
+    return dates;
+  }
+
+  /**
+   * Get the start of the week for a given date
+   * @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;
+  }
+
+  /**
+   * Get the end of the week for a given date
+   * @param date - Any date in the week
+   * @param firstDayOfWeek - 0 for Sunday, 1 for Monday
+   * @returns The end date of the week
+   */
+  getWeekEnd(date: Date, firstDayOfWeek: number = 1): Date {
+    const weekStart = this.getWeekStart(date, firstDayOfWeek);
+    const weekEnd = new Date(weekStart);
+    weekEnd.setDate(weekStart.getDate() + 6);
+    weekEnd.setHours(23, 59, 59, 999);
+    return weekEnd;
+  }
+
+  /**
+   * Get week number for a date (ISO 8601)
+   * @param date - The date to get week number for
+   * @returns Week number (1-53)
+   */
+  getWeekNumber(date: Date): number {
+    const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
+    const dayNum = d.getUTCDay() || 7;
+    d.setUTCDate(d.getUTCDate() + 4 - dayNum);
+    const yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
+    return Math.ceil((((d.getTime() - yearStart.getTime()) / 86400000) + 1)/7);
+  }
+
+  /**
+   * Format a date range for display
+   * @param start - Start date
+   * @param end - End date
+   * @returns Formatted date range string
+   */
+  formatDateRange(start: Date, end: Date): string {
+    const months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
+    
+    const startMonth = months[start.getMonth()];
+    const endMonth = months[end.getMonth()];
+    const startDay = start.getDate();
+    const endDay = end.getDate();
+    const startYear = start.getFullYear();
+    const endYear = end.getFullYear();
+    
+    if (startYear !== endYear) {
+      return `${startMonth} ${startDay}, ${startYear} - ${endMonth} ${endDay}, ${endYear}`;
+    } else if (startMonth === endMonth) {
+      return `${startMonth} ${startDay} - ${endDay}, ${startYear}`;
+    } else {
+      return `${startMonth} ${startDay} - ${endMonth} ${endDay}, ${startYear}`;
+    }
+  }
+
+  /**
+   * Format a date to ISO date string (YYYY-MM-DD)
+   * @param date - Date to format
+   * @returns ISO date string
+   */
+  formatISODate(date: Date): string {
+    return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
+  }
+
+  /**
+   * Check if a date is today
+   * @param date - Date to check
+   * @returns True if the date is today
+   */
+  isToday(date: Date): boolean {
+    const today = new Date();
+    return date.toDateString() === today.toDateString();
+  }
+
+  /**
+   * Add days to a date
+   * @param date - Base date
+   * @param days - Number of days to add (can be negative)
+   * @returns New date
+   */
+  addDays(date: Date, days: number): Date {
+    const result = new Date(date);
+    result.setDate(result.getDate() + days);
+    return result;
+  }
+
+  /**
+   * Add weeks to a date
+   * @param date - Base date
+   * @param weeks - Number of weeks to add (can be negative)
+   * @returns New date
+   */
+  addWeeks(date: Date, weeks: number): Date {
+    return this.addDays(date, weeks * 7);
+  }
+
+  /**
+   * Get all dates in a week
+   * @param weekStart - Start of the week
+   * @returns Array of 7 dates for the full week
+   */
+  getFullWeekDates(weekStart: Date): Date[] {
+    const dates: Date[] = [];
+    for (let i = 0; i < 7; i++) {
+      dates.push(this.addDays(weekStart, i));
+    }
+    return dates;
+  }
+
+  /**
+   * Get the day name for a date
+   * @param date - Date to get day name for
+   * @param format - 'short' or 'long'
+   * @returns Day name
+   */
+  getDayName(date: Date, format: 'short' | 'long' = 'short'): string {
+    const days = {
+      short: ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'],
+      long: ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
+    };
+    return days[format][date.getDay()];
+  }
+}
+
+// Create singleton instance with config
+export function createDateCalculator(config: CalendarConfig): DateCalculator {
+  return new DateCalculator(config);
+}
\ No newline at end of file