Documentation of days spent physically outside the United States during the earlier assignment: passport pages and stamps, entry and exit records, boarding passes, employer travel logs and calendars. The analysis is only as good as the day-level evidence supporting it.
Reconstruct the calendar while the sources still exist
This is tedious work and it rewards patience. Begin with the passport covering the period and transcribe every stamp with its date. Fill the gaps with airline records, which many carriers retain for a limited time, and with expense claims and calendars from the employer if the person is still on good terms with it.
Where a period cannot be evidenced, it is safer to leave it out of the calculation than to estimate, because an estimate that later proves wrong undermines the whole submission. Give the assembled record to counsel in a simple dated list with the supporting documents attached, and let them decide what can properly be claimed. Where a gap simply cannot be closed, say so plainly rather than filling it in with a plausible date.
Hypothetical example: Hana maintains the proprietary recipe-control system used by a specialty tea bottler and will deploy it for a U.S. affiliate. The first review should create a document trail from corporate ownership, to her foreign employment, to the internal system she knows.
Version histories, restricted manuals, training records and access controls can show that the knowledge belongs to the enterprise. Ask the business to preserve contemporaneous records rather than create a glossy summary after the fact. A weak file relies on a title such as systems specialist but never describes the actual process, how scarce the knowledge is, or how the destination role will use it.