29 C.F.R. § 516.6 : Labor — REGULATIONS RELATING TO LABOR — WAGE AND HOUR DIVISION, DEPARTMENT OF LABOR — REGULATIONS — RECORDS TO BE KEPT BY EMPLOYERS —Introductory — GENERAL REQUIREMENTS — Records to be preserved 2 years

(a) Supplementary basic records: Each employer required to maintain records under this part shall preserve for a period of at least 2 years.
(1) Basic employment and earnings records. From the date of last entry, all basic time and earning cards or sheets on which are entered the daily starting and stopping time of individual employees, or of separate work forces, or the amounts of work accomplished by individual employees on a daily, weekly, or pay period basis (for example, units produced) when those amounts determine in whole or in part the pay period earnings or wages of those employees.
(2) Wage rate tables. From their last effective date, all tables or schedules of the employer which provide the piece rates or other rates used in computing straight-time earnings, wages, or salary, or overtime pay computation.
(b) Order, shipping, and billing records: From the last date of entry, the originals or true copies of all customer orders or invoices received, incoming or outgoing shipping or delivery records, as well as all bills of lading and all billings to customers (not including individual sales slips, cash register tapes or the like) which the employer retains or makes in the usual course of business operations.
(c) Records of additions to or deductions from wages paid:
(1) Those records relating to individual employees referred to in §516.2(a)(10) and
(2) All records used by the employer in determining the original cost, operating and maintenance cost, and depreciation and interest charges, if such costs and charges are involved in the additions to or deductions from wages paid.

29 C.F.R. §516.6