4 C.F.R. § 21.2 : Accounts — GOVERNMENT ACCOUNTABILITY OFFICE — GENERAL PROCEDURES — BID PROTEST REGULATIONS — Time for filing

(a)
(1) Protests based upon alleged improprieties in a solicitation which are apparent prior to bid opening or the time set for receipt of initial proposals shall be filed prior to bid opening or the time set for receipt of initial proposals. In procurements where proposals are requested, alleged improprieties which do not exist in the initial solicitation but which are subsequently incorporated into the solicitation must be protested not later than the next closing time for receipt of proposals following the incorporation. If no closing time has been established, or if no further submissions are anticipated, any alleged solicitation improprieties must be protested within 10 days of when the alleged impropriety was known or should have been known.
(2) Protests other than those covered by paragraph (a)(1) of this section shall be filed not later than 10 days after the basis of protest is known or should have been known (whichever is earlier), with the exception of protests challenging a procurement conducted on the basis of competitive proposals under which a debriefing is requested and, when requested, is required. In such cases, with respect to any protest basis which is known or should have been known either before or as a result of the debriefing, and which does not involve an alleged solicitation impropriety covered by paragraph (a)(1) of this section, the initial protest shall not be filed before the debriefing date offered to the protester, but shall be filed not later than 10 days after the date on which the debriefing is held.
(3) If a timely agency-level protest was previously filed, any subsequent protest to GAO must be filed within 10 days of actual or constructive knowledge of initial adverse agency action, provided the agency-level protest was filed in accordance with paragraphs (a)(1) and (2) of this section, unless the agency imposes a more stringent time for filing, in which case the agency's time for filing will control. In cases where an alleged impropriety in a solicitation is timely protested to an agency, any subsequent protest to GAO will be considered timely if filed within the 10-day period provided by this paragraph, even if filed after bid opening or the closing time for receipt of proposals.
(b) Protests untimely on their face may be dismissed. A protester shall include in its protest all information establishing the timeliness of the protest; a protester will not be permitted to introduce for the first time in a request for reconsideration information necessary to establish that the protest was timely.
(c) GAO, for good cause shown, or where it determines that a protest raises issues significant to the procurement system, may consider an untimely protest.

4 C.F.R. § 21.2

83 FR 13823, 5/1/2018