Unless an agency designates different business hours, the FAR says that a government agency is deemed to close at 4:30 p.m. local time–not 5:00 p.m., as it would be easy to assume.
In a recent case, the 4:30 p.m. closing time cost an unsuccessful offeror a chance at a GAO protest because the offeror’s debriefing request, sent to the agency at 4:59 p.m., was deemed untimely.