feat: improve date handling by introducing utility functions for API datetime parsing and formatting

frontend/src/lib/datetime.ts

@@ -0,0 +1,53 @@
+const HAS_TZ_RE = /[zZ]|[+-]\d\d:\d\d$/;
+const DATE_ONLY_RE = /^\d{4}-\d{2}-\d{2}$/;
+
+/**
+ * Backend timestamps are emitted as ISO strings (often without a timezone).
+ * Treat missing timezone info as UTC, then format in the browser's local timezone.
+ */
+export function normalizeApiDatetime(value: string): string {
+  const trimmed = value.trim();
+  if (!trimmed) return trimmed;
+  if (DATE_ONLY_RE.test(trimmed)) {
+    // Convert date-only to a valid ISO timestamp.
+    return `${trimmed}T00:00:00Z`;
+  }
+  return HAS_TZ_RE.test(trimmed) ? trimmed : `${trimmed}Z`;
+}
+
+export function parseApiDatetime(value?: string | null): Date | null {
+  if (!value) return null;
+  const normalized = normalizeApiDatetime(value);
+  const date = new Date(normalized);
+  if (Number.isNaN(date.getTime())) return null;
+  return date;
+}
+
+export function apiDatetimeToMs(value?: string | null): number | null {
+  const date = parseApiDatetime(value);
+  return date ? date.getTime() : null;
+}
+
+export function toLocalDateInput(value?: string | null): string {
+  const date = parseApiDatetime(value);
+  if (!date) return "";
+  const year = String(date.getFullYear());
+  const month = String(date.getMonth() + 1).padStart(2, "0");
+  const day = String(date.getDate()).padStart(2, "0");
+  return `${year}-${month}-${day}`;
+}
+
+export function localDateInputToUtcIso(value?: string | null): string | null {
+  if (!value) return null;
+  const trimmed = value.trim();
+  if (!trimmed) return null;
+  const match = trimmed.match(/^(\d{4})-(\d{2})-(\d{2})$/);
+  if (!match) return null;
+  const year = Number(match[1]);
+  const monthIndex = Number(match[2]) - 1;
+  const day = Number(match[3]);
+  const date = new Date(year, monthIndex, day);
+  if (Number.isNaN(date.getTime())) return null;
+  return date.toISOString();
+}
+