A desired date can be discussed, but do not rely on it as an eligibility conclusion. Have the actual qualifying period and process assessed first.
Keep commitments conditional on the analysis
Identify uncertain periods and any upcoming travel that may affect the facts. Explain changes before relying on the earlier calculation. A business deadline cannot substitute for the required foreign employment or other L conditions.
A desired date usually originates with someone who is not carrying the eligibility risk. A U.S. manager needs the role filled and a client expects a project to begin, and neither is the party whose status depends on the analysis being right.
Let the applicant hold that line: agree that any date given to a hiring manager is provisional until the qualifying period has been assessed, and say so in writing when the date is first offered. Then keep the obligation running in both directions. The applicant reports new travel or a change of duties as it happens; the adviser reports when a reconciliation changes the answer.
Hypothetical example: an arboriculture firm's project schedule assumes a June start, and the applicant tells the project lead in writing that June remains subject to the review rather than letting the schedule harden around it. A commitment made before the days are reconciled does not become safer as the date approaches, and L-1B classification carries a five-year maximum, which is a further reason the actual dates matter more than the intended ones.