Start with passports and available electronic arrival records, then fill gaps using boarding passes, calendars, payroll records, expense claims and old approval notices. Where a stamp is missing or illegible, corroborate the date from two independent sources and note the method used to reach it.
Document the gaps as carefully as the entries
A chronology with an honest note reading "no record for this period, reconstructed from payroll" is more useful than a confident line with nothing behind it. Keep the source beside each date. Former employers, expired passports and previous representatives are all worth contacting early, since responses take time.
Assemble it as one dated file counsel can review without interviewing the worker about every entry. Add one source that people overlook because it feels bureaucratic rather than evidential: their own travel records. Boarding passes, expired passports with their stamps, calendar entries, expense claims and old itineraries between them reconstruct periods that no employer's file records, and they are frequently the only material covering time spent outside the country.
Gather them alongside the payroll and personnel records rather than afterwards, and note against each entry what it is based on. Hypothetical example: a worker rebuilding six years of history finds an eleven-week period nobody can account for from employment records, and an expired passport and a set of expense claims together establish where the person was, with the chronology recording precisely that basis rather than presenting the dates as though a personnel file had supplied them.