It can inform planning, but it does not control the U.S. immigration decision or authorize work.
Build a conditional sequence
Confirm how the employer will handle a changed start and what activities are expected in the meantime. Have changes to duties or employment reviewed. Keep resignation, travel, admission and commencement of work as distinct events rather than treating a single calendar date as approval of them all.
Obtain the final employer letter only after the role, supervisor, pay, location, and expected duration are settled; then check whether the degree, licence, and citizenship documents still match that version. Once a turnaround date is confirmed with the U.S. facility, the support letter, credential evidence and duty description should be finalized well before travel, so the officer reviewing the case at entry has a complete file rather than a promise that documents will follow. Meetings booked around an assumed start date create pressure to proceed on assumptions, and moving a calendar entry is far easier than undoing an activity already performed. Ordering official copies from an institution or professional order and arranging certified translation both take time that is difficult to compress, and neither should be squeezed against a start date already promised to an employer.