The petition should be filed only when the continuous year is complete, counted from the start of qualifying work and excluding U.S. days. The plant's opening date does not move that calculation.
Let the chronology set the filing date
Take the date qualifying managerial work began, add twelve months, then push the date forward by the number of days spent in the United States for the group. That is the earliest filing date for the petition. Between now and then the manager can continue commissioning visits in an appropriate status, but every additional U.S.
day pushes the L-1A date back by one day, so the employer may prefer to send someone else for the remaining trips. If the U.S. plant will not be operating by the filing date, the petition is a new-office case with a one-year initial approval and an extension that requires evidence the plant is doing business.
The family's L-2 admission follows the principal's, so leases and school enrolments should be dated from the confirmed approval or port-of-entry admission, not from the plant's target opening.