Tell the reviewer what is missing and what other genuine evidence exists. Do not replace an unavailable record with an invented account.
Assess the gap before relying on the timetable
Explain what the record was expected to establish and who may be able to confirm the fact. The effect depends on the actual evidence and requirements. A preferred start date cannot make an unsupported knowledge claim sufficient.
The instinct when a record cannot be found is to fill the space, and that is the failure. A summary is written from memory, described as reconstructed from company records, and it now has to be defended. Say instead exactly what is missing, what it would have shown, who held it and why it is unavailable — a retention schedule that expired, a system decommissioned in a merger, a supplier that no longer exists.
Then look for the fact elsewhere: a colleague who can attest from direct knowledge, a customer acceptance signed at the time, an expense or travel record placing the applicant on the site. Ask what the evidence actually available establishes and what remains open, and let the assessment run on that. A start date the business prefers cannot make an unsupported claim sufficient, and a candid gap is easier to answer than a confident one.