Accounts receivable as of December 31, 2021 and 2020, consist of the following:

Billed receivables from the federal government include amounts due from both prime contracts and subcontracts where the federal government is the end customer. Unbilled receivables include short-term contract assets where billing cycles differ from calendar months, or a monthly fixed billing amount does not reflect the revenue earned in a given month. The accounts receivable balance as of December 31, 2019, was $668,712.