lib/dateRange.ts

/**
 * `new Date("YYYY-MM-DD")` parses the string as UTC midnight, but a later
 * `.setHours()` call always applies in the server's local timezone. On a
 * non-UTC server (e.g. IST, UTC+5:30) that mismatch silently shifts the
 * boundary by the offset - excluding same-day data for hours after local
 * midnight. Parsing the components directly keeps everything local.
 */
function parseLocalDate(dateStr: string): Date {
  const [y, m, d] = dateStr.split('-').map(Number)
  return new Date(y, m - 1, d)
}

// A full ISO timestamp (recency-window presets like "last 15 minutes" send one)
// carries its own time-of-day and should be used exactly as-is, not snapped to a
// day boundary the way a bare "YYYY-MM-DD" is.
function isExactInstant(dateStr: string): boolean {
  return dateStr.includes('T')
}

export function startOfDay(dateStr: string | null | undefined, fallback: Date): Date {
  if (dateStr && isExactInstant(dateStr)) return new Date(dateStr)
  const d = dateStr ? parseLocalDate(dateStr) : fallback
  d.setHours(0, 0, 0, 0)
  return d
}

export function endOfDay(dateStr: string | null | undefined, fallback: Date): Date {
  if (dateStr && isExactInstant(dateStr)) return new Date(dateStr)
  const d = dateStr ? parseLocalDate(dateStr) : fallback
  d.setHours(23, 59, 59, 999)
  return d
}