Your I-94 arrival and departure history, passport pages with stamps and visas, tickets or boarding passes, any correspondence from a US agency, and copies of applications you previously signed. Together these fix dates. An unsupported recollection may be insufficient, and a gap in the record is itself something to resolve deliberately.
Where the paper disagrees with the memory
Suppose a government record shows a departure in March while a boarding pass shows February. Do not simply repeat whichever version is convenient, and do not assume the official entry must be right; notices can contain errors. Document both, note the source of each, and have the discrepancy examined before it is restated in a filing. Consistency across everything you submit is what makes the chronology credible.
A private invoice or reimbursement receipt may not prove that processing or an instructed gift was paid to the proper programme recipient. A first review should assemble the applicable EB eligibility evidence, identity and family records, verified source documents, and the official payment instruction. Keep a dated copy of the official page with the figures as they appear on the day, retain payment records, and respond to the instructions actually issued.