box-nestjs-monorepo/apps/box-stats-api/src/feature/time-helper.ts

// box-stats-api/src/feature/time-helper.ts
const TZ_OFFSET_SEC = 8 * 3600;

export function getGmt8HourStartUtcSec(epochSec: number): number {
  const shifted = epochSec + TZ_OFFSET_SEC;
  const hourStartShifted = Math.floor(shifted / 3600) * 3600;
  return hourStartShifted - TZ_OFFSET_SEC; // still UTC epoch seconds
}

export function getGmt8CurrentHourStartUtcSec(nowSec: number): number {
  return getGmt8HourStartUtcSec(nowSec);
}

export function computeHourlyWindowUtcSec(nowSec: number, lookbackHours = 2) {
  const currentHourStartUtc = getGmt8CurrentHourStartUtcSec(nowSec);
  const startSec = currentHourStartUtc - lookbackHours * 3600;
  const endSec = nowSec; // realtime
  return { startSec, endSec };
}