Only once the applicable authorization is in place for the applicant's route. A filed application does not by itself permit work, and an employer's chosen start date does not change that. The training weeks are part of the employment, not a period outside it. Two calendars are usually in play: the authorization date, and the client programme the employer has already committed to. Only the first governs when work may lawfully begin, however firmly the second has been sold to a customer.
Make the start date conditional, then verify
Ask the employer to record the start date as subject to authorization, so a scheduling change does not become pressure to begin early. After entry, retrieve the admission record and check the classification and dates rather than assuming they match what was requested. If anything is wrong, raise it promptly with counsel instead of working through it.
Ask to see the project programme or delivery schedule the employer is working to. Where it shows a milestone falling before any plausible authorization date, the pressure to start early is structural rather than personal, and naming it at the outset is easier than resisting it week by week. A written note recording that the assignment cannot be resourced by this person until authorization exists puts the scheduling problem back with the party that created it.
Moving a milestone is an ordinary commercial adjustment; working without authorization is not.