insights-correlations (#22)

Reviewed-on: #22

spa/src/lib/date.ts

@@ -18,18 +18,34 @@ export function shortDate(dateStr: string): string {
   }
 }
 
-export function parseLocalDate(s: string): Date {
-  const [datePart, timePart] = s.split("T")
-  if (!datePart) return new Date()
-  const [y, m, d] = datePart.split("-").map(Number)
-  if (timePart) {
-    const [h, min, sec] = timePart.split(":").map(Number)
-    return new Date(y!, m! - 1, d!, h, min, sec)
-  }
-  return new Date(y!, m! - 1, d!)
+// `watched_at` is stored as naive UTC ("YYYY-MM-DDTHH:MM:SS", no zone).
+// Date-only imports carry no time and are stored at exactly 00:00:00; those
+// are calendar dates, not instants, and must not be shifted between zones.
+
+export function isDateOnly(s: string): boolean {
+  return /^\d{4}-\d{2}-\d{2}([T ]00:00(:00)?)?$/.test(s)
 }
 
-export function formatLocalDateTime(d: Date): string {
-  const pad = (n: number) => n.toString().padStart(2, "0")
-  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}:${pad(d.getSeconds())}`
+export function parseWatchedAt(s: string): Date {
+  const [y, m, d] = s.slice(0, 10).split("-").map(Number)
+  if (isDateOnly(s)) return new Date(y!, m! - 1, d!)
+  const parsed = new Date(`${s.replace(" ", "T")}Z`)
+  return Number.isNaN(parsed.getTime()) ? new Date() : parsed
+}
+
+export function formatWatchedAt(d: Date, dateOnly = false): string {
+  const pad = (n: number) => n.toString().padStart(2, "0")
+  if (dateOnly) {
+    return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T00:00:00`
+  }
+  return d.toISOString().split(".")[0]!
+}
+
+// Swap the calendar day of `current` for `picked` (the date picker yields local
+// midnight), keeping the time of day so the stored UTC instant stays on the
+// chosen day.
+export function withPickedDay(current: Date, picked: Date): Date {
+  const next = new Date(current)
+  next.setFullYear(picked.getFullYear(), picked.getMonth(), picked.getDate())
+  return next
 }