Fix date offset bugs in calendar drag&drop and task creation

[Copilot]

Problem

The application had date offset bugs where tasks were being created 1 day earlier than intended in two scenarios:

1. Calendar drag & drop: When dragging tasks to specific calendar dates, tasks were created on the previous day
2. New task creation: When selecting dates for new tasks, they were saved one day before the selected date

Root Cause

The issue was caused by timezone handling problems when converting between JavaScript Date objects and date strings:

// Problematic code that caused date shifts:
const newStartDate = event.start.toISOString().split('T')[0];
const taskDate = dates[0].toISOString().split('T')[0];

When Date objects are created from date-only strings (like "2024-01-15"), JavaScript interprets them as UTC midnight. In timezones behind UTC, calling toISOString() would shift the date to the previous day.

Solution

Added timezone-safe utility functions and updated all date handling code:

New Utility Functions

function getLocalDateString(date) {
    // Safely converts any date input to YYYY-MM-DD format in local timezone
    if (!date) return new Date().toLocaleDateString('sv-SE');
    
    if (typeof date === 'string') {
        if (/^\d{4}-\d{2}-\d{2}$/.test(date)) return date;
        date = new Date(date + 'T00:00:00'); // Local time interpretation
    }
    
    if (date instanceof Date && !isNaN(date)) {
        return date.toLocaleDateString('sv-SE'); // sv-SE gives YYYY-MM-DD format
    }
    
    return new Date().toLocaleDateString('sv-SE');
}

function createLocalDate(dateString) {
    // Creates local Date objects from YYYY-MM-DD strings without timezone shifts
    if (/^\d{4}-\d{2}-\d{2}$/.test(dateString)) {
        return new Date(dateString + 'T00:00:00'); // Force local interpretation
    }
    return new Date(dateString + 'T00:00:00');
}

Fixed Functions

Calendar Drag & Drop:

// Before (buggy):
const newStartDate = event.start.toISOString().split('T')[0];
const newEndDate = new Date(event.end.getTime() - 86400000).toISOString().split('T')[0];

// After (fixed):
const newStartDate = getLocalDateString(event.start);
let newEndDate = newStartDate;
if (event.end) {
    const endDate = new Date(event.end);
    endDate.setDate(endDate.getDate() - 1);
    newEndDate = getLocalDateString(endDate);
}

Task Creation:

// Before (buggy):
startDate: dates[0].toISOString().split('T')[0],
endDate: (dates[1] || dates[0]).toISOString().split('T')[0],

// After (fixed):
startDate: getLocalDateString(dates[0]),
endDate: getLocalDateString(dates[1] || dates[0]),

Calendar Rendering:

// Before (buggy):
const exclusiveEnd = new Date(task.endDate || task.startDate);
exclusiveEnd.setDate(exclusiveEnd.getDate() + 1);
end: exclusiveEnd.toISOString().split('T')[0],

// After (fixed):
const startDate = createLocalDate(task.startDate);
const endDate = createLocalDate(task.endDate || task.startDate);
const exclusiveEndDate = new Date(endDate);
exclusiveEndDate.setDate(exclusiveEndDate.getDate() + 1);
end: getLocalDateString(exclusiveEndDate),

Testing

Created comprehensive tests covering:

• ✅ Date string consistency across timezone conversions
• ✅ Calendar drag & drop date preservation
• ✅ Task creation date accuracy
• ✅ Date round-trip consistency
• ✅ Compatibility with existing task data

All tests pass, confirming the fixes work correctly across different timezones without breaking existing functionality.

Impact

• Calendar drag & drop: Now preserves exact intended dates
• Task creation: User-selected dates are saved correctly
• Existing tasks: Remain unaffected by the changes
• Cross-timezone: Works consistently regardless of user timezone

Fixes #[issue-number] (カレンダーの日付オフセットバグ修正)

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