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frontend/src/utils/dates.ts

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+/**
+ * Date formatting and calculation utilities for Swedish locale (sv-SE).
+ * All functions use ISO 8601 format for input (YYYY-MM-DD) and Intl API for display.
+ */
+
+/**
+ * Format a date for display (e.g. "1 juni 2026" in Swedish).
+ * Accepts ISO string or Date object.
+ */
+export function formatDateForDisplay(date: string | Date): string {
+  const d = typeof date === 'string' ? new Date(date + 'T00:00:00Z') : date
+  if (!isValidDate(d)) return ''
+  const formatter = new Intl.DateTimeFormat('sv-SE', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })
+  return formatter.format(d)
+}
+
+/**
+ * Format a date for HTML input value (e.g. "2026-06-01").
+ * Accepts ISO string or Date object.
+ */
+export function formatDateForInput(date: string | Date): string {
+  const d = typeof date === 'string' ? new Date(date + 'T00:00:00Z') : date
+  if (!isValidDate(d)) return ''
+  const year = d.getUTCFullYear()
+  const month = String(d.getUTCMonth() + 1).padStart(2, '0')
+  const day = String(d.getUTCDate()).padStart(2, '0')
+  return `${year}-${month}-${day}`
+}
+
+/**
+ * Parse an ISO date string (YYYY-MM-DD) to a Date object.
+ */
+export function parseInputDate(value: string): Date {
+  return new Date(value + 'T00:00:00Z')
+}
+
+/**
+ * Type guard: check if a value is a valid date.
+ */
+export function isValidDate(date: any): boolean {
+  return date instanceof Date && !isNaN(date.getTime())
+}
+
+/**
+ * Get the Swedish day name (e.g. "Fredag").
+ */
+export function getDayName(date: Date): string {
+  if (!isValidDate(date)) return ''
+  const formatter = new Intl.DateTimeFormat('sv-SE', { weekday: 'long' })
+  return formatter.format(date)
+}
+
+/**
+ * Get the Swedish month name (e.g. "Juni").
+ */
+export function getMonthName(date: Date): string {
+  if (!isValidDate(date)) return ''
+  const formatter = new Intl.DateTimeFormat('sv-SE', { month: 'long' })
+  return formatter.format(date)
+}

frontend/src/utils/formatting.ts

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+/**
+ * Formatting utilities for Swedish locale (sv-SE).
+ */
+
+/**
+ * Format a number of days with Swedish plural rules (1 dag, 2 dagar, etc.).
+ */
+export function formatDayCount(days: number): string {
+  if (days === 1) return '1 dag'
+  return `${days} dagar`
+}
+
+/**
+ * Format a percentage from numerator and denominator (e.g. "75%").
+ * Rounds to nearest integer.
+ */
+export function formatPercentage(numerator: number, denominator: number): string {
+  if (denominator === 0) return '0%'
+  const percentage = Math.round((numerator / denominator) * 100)
+  return `${percentage}%`
+}