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/api.js

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+/**
+ * API Functions
+ * Backend communication layer
+ */
+
+/**
+ * Fetch entries from backend
+ */
+async function fetchEntries(fromDate = null, toDate = null) {
+  try {
+    let url = '/api/entries';
+    const params = new URLSearchParams();
+    
+    if (fromDate) params.append('from', fromDate);
+    if (toDate) params.append('to', toDate);
+    
+    if (params.toString()) {
+      url += '?' + params.toString();
+    }
+    
+    const response = await fetch(url);
+    
+    if (!response.ok) {
+      throw new Error('Failed to fetch entries');
+    }
+    
+    const entries = await response.json();
+    return entries;
+  } catch (error) {
+    console.error('Error fetching entries:', error);
+    showNotification('Fehler beim Laden der Einträge', 'error');
+    return [];
+  }
+}
+
+/**
+ * Create a new entry
+ */
+async function createEntry(date, startTime, endTime, pauseMinutes, location) {
+  try {
+    const body = {
+      date: formatDateISO(date),
+      startTime,
+      endTime,
+      location: location || 'office'
+    };
+    
+    // Only include pauseMinutes if explicitly provided (not empty)
+    if (pauseMinutes !== null && pauseMinutes !== undefined && pauseMinutes !== '') {
+      body.pauseMinutes = parseInt(pauseMinutes);
+    }
+    
+    const response = await fetch('/api/entries', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json'
+      },
+      body: JSON.stringify(body)
+    });
+    
+    if (!response.ok) {
+      const error = await response.json();
+      throw new Error(error.error || 'Failed to create entry');
+    }
+    
+    const entry = await response.json();
+    return entry;
+  } catch (error) {
+    console.error('Error creating entry:', error);
+    showNotification(error.message || 'Fehler beim Erstellen des Eintrags', 'error');
+    return null;
+  }
+}
+
+/**
+ * Update an existing entry
+ */
+async function updateEntry(id, date, startTime, endTime, pauseMinutes, location) {
+  try {
+    const body = {
+      date: formatDateISO(date),
+      startTime,
+      endTime,
+      location: location || 'office'
+    };
+    
+    // Only include pauseMinutes if explicitly provided (not empty)
+    if (pauseMinutes !== null && pauseMinutes !== undefined && pauseMinutes !== '') {
+      body.pauseMinutes = parseInt(pauseMinutes);
+    }
+    
+    const response = await fetch(`/api/entries/${id}`, {
+      method: 'PUT',
+      headers: {
+        'Content-Type': 'application/json'
+      },
+      body: JSON.stringify(body)
+    });
+    
+    if (!response.ok) {
+      const error = await response.json();
+      throw new Error(error.error || 'Failed to update entry');
+    }
+    
+    const entry = await response.json();
+    return entry;
+  } catch (error) {
+    console.error('Error updating entry:', error);
+    showNotification(error.message || 'Fehler beim Aktualisieren des Eintrags', 'error');
+    return null;
+  }
+}
+
+/**
+ * Delete an entry
+ */
+async function deleteEntry(id) {
+  try {
+    const response = await fetch(`/api/entries/${id}`, {
+      method: 'DELETE'
+    });
+    
+    if (!response.ok) {
+      throw new Error('Failed to delete entry');
+    }
+    
+    return true;
+  } catch (error) {
+    console.error('Error deleting entry:', error);
+    showNotification('Fehler beim Löschen des Eintrags', 'error');
+    return false;
+  }
+}
+
+/**
+ * Get a setting by key
+ */
+async function getSetting(key) {
+  try {
+    const response = await fetch(`/api/settings/${key}`);
+    
+    if (!response.ok) {
+      if (response.status === 404) {
+        return null; // Setting doesn't exist yet
+      }
+      throw new Error('Failed to get setting');
+    }
+    
+    const data = await response.json();
+    return data.value;
+  } catch (error) {
+    console.error('Error getting setting:', error);
+    return null;
+  }
+}
+
+/**
+ * Set a setting
+ */
+async function setSetting(key, value) {
+  try {
+    const response = await fetch('/api/settings', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json'
+      },
+      body: JSON.stringify({ key, value })
+    });
+    
+    if (!response.ok) {
+      throw new Error('Failed to set setting');
+    }
+    
+    return true;
+  } catch (error) {
+    console.error('Error setting setting:', error);
+    showNotification('Fehler beim Speichern der Einstellung', 'error');
+    return false;
+  }
+}

public/js/holidays.js

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+/**
+ * Holiday Functions
+ * German public holidays calculation and checking
+ */
+
+/**
+ * Check if date is weekend
+ */
+function isWeekend(date) {
+  const day = date.getDay();
+  return day === 0 || day === 6; // Sunday or Saturday
+}
+
+/**
+ * Calculate Easter Sunday for a given year (Gauss algorithm)
+ */
+function getEasterSunday(year) {
+  const a = year % 19;
+  const b = Math.floor(year / 100);
+  const c = year % 100;
+  const d = Math.floor(b / 4);
+  const e = b % 4;
+  const f = Math.floor((b + 8) / 25);
+  const g = Math.floor((b - f + 1) / 3);
+  const h = (19 * a + b - d - g + 15) % 30;
+  const i = Math.floor(c / 4);
+  const k = c % 4;
+  const l = (32 + 2 * e + 2 * i - h - k) % 7;
+  const m = Math.floor((a + 11 * h + 22 * l) / 451);
+  const month = Math.floor((h + l - 7 * m + 114) / 31);
+  const day = ((h + l - 7 * m + 114) % 31) + 1;
+  return new Date(year, month - 1, day);
+}
+
+/**
+ * Get all public holidays for a given year and Bundesland
+ */
+function getPublicHolidays(year, bundesland) {
+  const holidays = [];
+  
+  // Fixed holidays (all states)
+  holidays.push({ date: new Date(year, 0, 1), name: 'Neujahr' });
+  holidays.push({ date: new Date(year, 4, 1), name: 'Tag der Arbeit' });
+  holidays.push({ date: new Date(year, 9, 3), name: 'Tag der Deutschen Einheit' });
+  holidays.push({ date: new Date(year, 11, 25), name: '1. Weihnachtstag' });
+  holidays.push({ date: new Date(year, 11, 26), name: '2. Weihnachtstag' });
+  
+  // Heilige Drei Könige (BW, BY, ST)
+  if (['BW', 'BY', 'ST'].includes(bundesland)) {
+    holidays.push({ date: new Date(year, 0, 6), name: 'Heilige Drei Könige' });
+  }
+  
+  // Internationaler Frauentag (BE, MV since 2023)
+  if (['BE'].includes(bundesland) || (bundesland === 'MV' && year >= 2023)) {
+    holidays.push({ date: new Date(year, 2, 8), name: 'Internationaler Frauentag' });
+  }
+  
+  // Weltkindertag (TH since 2019)
+  if (bundesland === 'TH' && year >= 2019) {
+    holidays.push({ date: new Date(year, 8, 20), name: 'Weltkindertag' });
+  }
+  
+  // Reformationstag (BB, MV, SN, ST, TH, + HB, HH, NI, SH since 2018)
+  const reformationstagStates = ['BB', 'MV', 'SN', 'ST', 'TH'];
+  if (year >= 2018) {
+    reformationstagStates.push('HB', 'HH', 'NI', 'SH');
+  }
+  if (reformationstagStates.includes(bundesland)) {
+    holidays.push({ date: new Date(year, 9, 31), name: 'Reformationstag' });
+  }
+  
+  // Allerheiligen (BW, BY, NW, RP, SL)
+  if (['BW', 'BY', 'NW', 'RP', 'SL'].includes(bundesland)) {
+    holidays.push({ date: new Date(year, 10, 1), name: 'Allerheiligen' });
+  }
+  
+  // Buß- und Bettag (only SN)
+  if (bundesland === 'SN') {
+    // Buß- und Bettag is the Wednesday before November 23
+    let bussbettag = new Date(year, 10, 23);
+    while (bussbettag.getDay() !== 3) { // 3 = Wednesday
+      bussbettag.setDate(bussbettag.getDate() - 1);
+    }
+    bussbettag.setDate(bussbettag.getDate() - 7); // One week before
+    holidays.push({ date: bussbettag, name: 'Buß- und Bettag' });
+  }
+  
+  // Easter-dependent holidays
+  const easter = getEasterSunday(year);
+  
+  // Karfreitag (Good Friday) - 2 days before Easter (all states)
+  const goodFriday = new Date(easter);
+  goodFriday.setDate(easter.getDate() - 2);
+  holidays.push({ date: goodFriday, name: 'Karfreitag' });
+  
+  // Ostermontag (Easter Monday) - 1 day after Easter (all states)
+  const easterMonday = new Date(easter);
+  easterMonday.setDate(easter.getDate() + 1);
+  holidays.push({ date: easterMonday, name: 'Ostermontag' });
+  
+  // Christi Himmelfahrt (Ascension Day) - 39 days after Easter (all states)
+  const ascension = new Date(easter);
+  ascension.setDate(easter.getDate() + 39);
+  holidays.push({ date: ascension, name: 'Christi Himmelfahrt' });
+  
+  // Pfingstmontag (Whit Monday) - 50 days after Easter (all states)
+  const whitMonday = new Date(easter);
+  whitMonday.setDate(easter.getDate() + 50);
+  holidays.push({ date: whitMonday, name: 'Pfingstmontag' });
+  
+  // Fronleichnam (Corpus Christi) - 60 days after Easter (BW, BY, HE, NW, RP, SL, + some communities in SN, TH)
+  if (['BW', 'BY', 'HE', 'NW', 'RP', 'SL'].includes(bundesland)) {
+    const corpusChristi = new Date(easter);
+    corpusChristi.setDate(easter.getDate() + 60);
+    holidays.push({ date: corpusChristi, name: 'Fronleichnam' });
+  }
+  
+  // Mariä Himmelfahrt (Assumption of Mary) - August 15 (BY in some communities, SL)
+  if (['SL'].includes(bundesland)) {
+    holidays.push({ date: new Date(year, 7, 15), name: 'Mariä Himmelfahrt' });
+  }
+  
+  return holidays;
+}
+
+/**
+ * Check if date is a public holiday
+ * Returns the holiday name or null
+ */
+function getHolidayName(date) {
+  const year = date.getFullYear();
+  const holidays = getPublicHolidays(year, currentBundesland);
+  
+  const dateStr = date.toISOString().split('T')[0];
+  const holiday = holidays.find(h => {
+    return h.date.toISOString().split('T')[0] === dateStr;
+  });
+  
+  return holiday ? holiday.name : null;
+}
+
+/**
+ * Check if date is a public holiday
+ */
+function isPublicHoliday(date) {
+  return getHolidayName(date) !== null;
+}
+
+/**
+ * Check if date is weekend or public holiday
+ */
+function isWeekendOrHoliday(date) {
+  return isWeekend(date) || isPublicHoliday(date);
+}

public/js/utils.js

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+/**
+ * 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];
+}