Add utility functions for date formatting, time rounding, and notifications

- Implemented functions to format dates between YYYY-MM-DD and DD.MM.YYYY
- Added functions to get today's date in ISO format
- Created functions to round time to the nearest 15 minutes
- Developed a function to format time as HH:MM
- Added a function to format duration in HH:MM:SS
- Implemented a toast notification system with auto-remove functionality
- Added functions to get day of the week and month names in German

public/js/utils.js

@@ -0,0 +1,126 @@
+/**
+ * Utility Functions
+ * Date/Time formatting, rounding, duration formatting
+ */
+
+/**
+ * Format date from YYYY-MM-DD to DD.MM.YYYY
+ */
+function formatDateDisplay(dateStr) {
+  const [year, month, day] = dateStr.split('-');
+  return `${day}.${month}.${year}`;
+}
+
+/**
+ * Format date from DD.MM.YYYY to YYYY-MM-DD
+ */
+function formatDateISO(dateStr) {
+  const [day, month, year] = dateStr.split('.');
+  return `${year}-${month}-${day}`;
+}
+
+/**
+ * Get today's date in YYYY-MM-DD format
+ */
+function getTodayISO() {
+  const today = new Date();
+  const year = today.getFullYear();
+  const month = String(today.getMonth() + 1).padStart(2, '0');
+  const day = String(today.getDate()).padStart(2, '0');
+  return `${year}-${month}-${day}`;
+}
+
+/**
+ * Round time down to nearest 15 minutes
+ */
+function roundDownTo15Min(date) {
+  const minutes = date.getMinutes();
+  const roundedMinutes = Math.floor(minutes / 15) * 15;
+  date.setMinutes(roundedMinutes);
+  date.setSeconds(0);
+  date.setMilliseconds(0);
+  return date;
+}
+
+/**
+ * Round time up to nearest 15 minutes
+ */
+function roundUpTo15Min(date) {
+  const minutes = date.getMinutes();
+  const roundedMinutes = Math.ceil(minutes / 15) * 15;
+  date.setMinutes(roundedMinutes);
+  date.setSeconds(0);
+  date.setMilliseconds(0);
+  return date;
+}
+
+/**
+ * Format time as HH:MM
+ */
+function formatTime(date) {
+  const hours = String(date.getHours()).padStart(2, '0');
+  const minutes = String(date.getMinutes()).padStart(2, '0');
+  return `${hours}:${minutes}`;
+}
+
+/**
+ * Format seconds to HH:MM:SS
+ */
+function formatDuration(seconds) {
+  const hrs = Math.floor(seconds / 3600);
+  const mins = Math.floor((seconds % 3600) / 60);
+  const secs = seconds % 60;
+  return `${String(hrs).padStart(2, '0')}:${String(mins).padStart(2, '0')}:${String(secs).padStart(2, '0')}`;
+}
+
+/**
+ * Show toast notification
+ */
+function showNotification(message, type = 'info') {
+  const container = document.getElementById('toastContainer');
+  
+  // Create toast element
+  const toast = document.createElement('div');
+  toast.className = `toast toast-${type}`;
+  
+  // Icon based on type
+  const icons = {
+    success: '✓',
+    error: '✕',
+    info: 'ℹ'
+  };
+  
+  toast.innerHTML = `
+    <span class="toast-icon">${icons[type] || 'ℹ'}</span>
+    <span>${message}</span>
+  `;
+  
+  container.appendChild(toast);
+  
+  // Auto-remove after 3 seconds
+  setTimeout(() => {
+    toast.classList.add('hiding');
+    setTimeout(() => {
+      container.removeChild(toast);
+    }, 300);
+  }, 3000);
+}
+
+/**
+ * Get day of week abbreviation in German
+ */
+function getDayOfWeek(date) {
+  const days = ['So', 'Mo', 'Di', 'Mi', 'Do', 'Fr', 'Sa'];
+  return days[date.getDay()];
+}
+
+/**
+ * Get month name in German
+ */
+function getMonthName(month) {
+  const months = [
+    'Januar', 'Februar', 'März', 'April', 'Mai', 'Juni',
+    'Juli', 'August', 'September', 'Oktober', 'November', 'Dezember'
+  ];
+  return months[month];
+}