Client start dates are commitments to a third party, which makes them the worst thing to build an immigration plan around. Get the offsite assessment done before agreeing dates, then work through preparation, any filing, a visa step where applicable, admission, and the lawful start of work in order.
Give the client a staged answer
Rather than a single start date, tell the client when the assessment will be complete and when a date can be confirmed. Plan how the early phase would run if the assignment is delayed — remote work from the current location, a colleague already authorized to work there, or a rescheduled phase. Confirm the qualifying employment period abroad early, since it can rule the plan out regardless of client urgency.
Add one further discipline to that staged answer: put the client's own approval times into the schedule explicitly. Where the file needs contractual material, a redaction decision, or a letter from the client, that request enters another organisation's legal queue and is subject to its priorities, its holidays and its own internal approvals. Ask for those items in the first week with a stated deadline and a named contact, and treat a non-response as a scheduling event rather than a background irritation.
Hypothetical example: a temperature-mapping validation specialist's employer requests client sign-off on a redacted statement of work in week one and receives it in week six, which is entirely survivable because the request went out first rather than after everything else was ready.