refactor: Update date utilities to use UTC methods consistently

Update remaining date helper functions to use UTC methods for
consistency with the timezone bug fixes. This ensures all date
comparisons and calculations work consistently across timezones.

Changes:
- Convert isSameDay() to use UTC date methods
- Update legacy recurrence date comparisons to use UTC
- Fix date arithmetic in default date generation
- Update date iteration loops to use UTC methods

These changes complement the timezone fix in Issue #314 by
ensuring all date utilities work consistently with UTC dates.

Author: callumalpass

src/utils/helpers.ts

@@ -184,11 +184,11 @@ export function calculateDefaultDate(defaultOption: 'none' | 'today' | 'tomorrow
 			break;
 		case 'tomorrow':
 			targetDate = new Date(today);
-			targetDate.setDate(today.getDate() + 1);
+			targetDate.setUTCDate(today.getUTCDate() + 1);
 			break;
 		case 'next-week':
 			targetDate = new Date(today);
-			targetDate.setDate(today.getDate() + 7);
+			targetDate.setUTCDate(today.getUTCDate() + 7);
 			break;
 		default:
 			return '';
@@ -198,12 +198,12 @@ export function calculateDefaultDate(defaultOption: 'none' | 'today' | 'tomorrow
 }
 
 /**
- * Checks if two dates are the same day
+ * Checks if two dates are the same day using UTC methods for consistency
  */
 export function isSameDay(date1: Date, date2: Date): boolean {
-	return date1.getFullYear() === date2.getFullYear() &&
-		date1.getMonth() === date2.getMonth() &&
-		date1.getDate() === date2.getDate();
+	return date1.getUTCFullYear() === date2.getUTCFullYear() &&
+		date1.getUTCMonth() === date2.getUTCMonth() &&
+		date1.getUTCDate() === date2.getUTCDate();
 }
 
 /**
@@ -354,9 +354,9 @@ export function isDueByRRule(task: TaskInfo, date: Date): boolean {
 	// Legacy recurrence object handling
 	const frequency = task.recurrence.frequency;
 	const targetDate = parseDate(formatUTCDateForCalendar(date));
-	const dayOfWeek = targetDate.getDay();
-	const dayOfMonth = targetDate.getDate();
-	const monthOfYear = targetDate.getMonth() + 1; // JavaScript months are 0-indexed
+	const dayOfWeek = targetDate.getUTCDay();
+	const dayOfMonth = targetDate.getUTCDate();
+	const monthOfYear = targetDate.getUTCMonth() + 1; // JavaScript months are 0-indexed
 	// Map JavaScript's day of week (0-6, where 0 is Sunday) to our day abbreviations
 	const weekdayMap = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat'];
 
@@ -382,8 +382,8 @@ export function isDueByRRule(task: TaskInfo, date: Date): boolean {
 			else if (task.due) {
 				try {
 					const originalDueDate = parseDate(task.due); // Safe parsing
-					return originalDueDate.getDate() === dayOfMonth && 
-							originalDueDate.getMonth() === targetDate.getMonth();
+					return originalDueDate.getUTCDate() === dayOfMonth && 
+							originalDueDate.getUTCMonth() === targetDate.getUTCMonth();
 				} catch (error) {
 					console.error(`Error parsing due date ${task.due}:`, error);
 					return false;
@@ -408,9 +408,9 @@ export function isRecurringTaskDueOn(task: any, date: Date): boolean {
 
 	const frequency = task.recurrence.frequency;
 	const targetDate = parseDate(formatUTCDateForCalendar(date));
-	const dayOfWeek = targetDate.getDay();
-	const dayOfMonth = targetDate.getDate();
-	const monthOfYear = targetDate.getMonth() + 1; // JavaScript months are 0-indexed
+	const dayOfWeek = targetDate.getUTCDay();
+	const dayOfMonth = targetDate.getUTCDate();
+	const monthOfYear = targetDate.getUTCMonth() + 1; // JavaScript months are 0-indexed
 	// Map JavaScript's day of week (0-6, where 0 is Sunday) to our day abbreviations
 	const weekdayMap = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat'];
 
@@ -436,8 +436,8 @@ export function isRecurringTaskDueOn(task: any, date: Date): boolean {
 			else if (task.due) {
 				try {
 					const originalDueDate = parseDate(task.due); // Safe parsing
-					return originalDueDate.getDate() === dayOfMonth && 
-						originalDueDate.getMonth() === targetDate.getMonth();
+					return originalDueDate.getUTCDate() === dayOfMonth && 
+						originalDueDate.getUTCMonth() === targetDate.getUTCMonth();
 				} catch (error) {
 					console.error(`Error parsing due date ${task.due}:`, error);
 					return false;
@@ -539,7 +539,7 @@ export function generateRecurringInstances(task: TaskInfo, startDate: Date, endD
 		if (isDueByRRule(task, current)) {
 			instances.push(new Date(current));
 		}
-		current.setDate(current.getDate() + 1);
+		current.setUTCDate(current.getUTCDate() + 1);
 	}
 
 	return instances;