Function computeNextCronRun

src/utils/cron.ts:125–187

(
  fields: CronFields,
  from: Date,
)

 * the second occurrence). This matches vixie-cron behavior.
 */
export function computeNextCronRun(
  fields: CronFields,
  from: Date,
): Date | null {
  const minuteSet = new Set(fields.minute)
  const hourSet = new Set(fields.hour)
  const domSet = new Set(fields.dayOfMonth)
  const monthSet = new Set(fields.month)
  const dowSet = new Set(fields.dayOfWeek)

  // Is the field wildcarded (full range)?
  const domWild = fields.dayOfMonth.length === 31
  const dowWild = fields.dayOfWeek.length === 7

  // Round up to the next whole minute (strictly after `from`)
  const t = new Date(from.getTime())
  t.setSeconds(0, 0)
  t.setMinutes(t.getMinutes() + 1)

  const maxIter = 366 * 24 * 60
  for (let i = 0; i < maxIter; i++) {
    const month = t.getMonth() + 1
    if (!monthSet.has(month)) {
      // Jump to start of next month
      t.setMonth(t.getMonth() + 1, 1)
      t.setHours(0, 0, 0, 0)
      continue
    }

    const dom = t.getDate()
    const dow = t.getDay()
    // When both dom/dow are constrained, either match is sufficient (OR semantics)
    const dayMatches =
      domWild && dowWild
        ? true
        : domWild
          ? dowSet.has(dow)
          : dowWild
            ? domSet.has(dom)
            : domSet.has(dom) || dowSet.has(dow)

    if (!dayMatches) {
      // Jump to start of next day
      t.setDate(t.getDate() + 1)
      t.setHours(0, 0, 0, 0)
      continue
    }

    if (!hourSet.has(t.getHours())) {
      t.setHours(t.getHours() + 1, 0, 0, 0)
      continue
    }

    if (!minuteSet.has(t.getMinutes())) {
      t.setMinutes(t.getMinutes() + 1)
      continue
    }