A defined term supports the temporary nature of the work, but it does not by itself set the period of admission or the validity of any approval, which are decided separately. Build the review schedule around when the agreements and duty documents will be final.
Keep the paperwork aligned as the term changes
A secondment term can be extended, shortened, or amended when a project shifts. If the term, location, or reporting line changes, the duty letter and the agreement should be updated together so they continue to describe the same arrangement. Treat the assignment start date in the agreement as an internal plan, not as the date on which work in the United States may lawfully begin.
A port-of-entry application is decided the same day; a USCIS petition has its own processing time. A renewed or extended secondment term inside a company's records is not the same event as a new or amended immigration filing, and treating the two as automatically aligned can leave a gap between what management has approved and what has actually been authorized. Ask whether an internal extension has been communicated to the person managing the immigration file, since a change agreed only at the business level does not update any record on its own. Keep a short written summary of each amendment, the reason for it, and the date it was agreed, so the history is available if a later reviewer needs to reconcile the internal term with the authorized period. Build in enough lead time for the immigration side to catch up whenever the business term changes.