Before the transfer is announced. If the secondment does not count, the qualifying year may have to be completed with the correct entity first, and that is a scheduling problem for the business rather than a paperwork problem for the individual.
A missing year cannot be compressed
This is the one requirement that no amount of good drafting can accelerate. If the records show that the year of qualifying employment outside the United States is not there within the look-back window, the options are to find a different arrangement, to wait until the year is complete, or to consider whether a different route fits. All three of those are business decisions that need lead time. Establish the position early, tell the American side what it means for the start date, and use any waiting period productively by assembling the specialized-knowledge documentation. The alternative — filing on an assumption about the secondment and learning otherwise months later — costs the same money and loses the time as well.
Hypothetical example: Alina, a textile-dye process specialist, has a foreign assignment interrupted by a short unpaid leave. Before choosing a filing date, the employer should compare payroll, leave approval, travel and assignment records to determine whether she has one continuous qualifying year abroad within the relevant three-year period. The analysis should distinguish a documented leave from a resignation or work for an unrelated employer rather than treating every blank pay period alike. Once the qualifying period is settled, the company can schedule the specialized-knowledge evidence and U.S. assignment. Prior L time must also be counted because L-1B classification is generally limited to five years.