Frequently, a party will require some action “by the close of business” (COB). What does that mean? While there does not appear to be a case in the United States dealing with this issue, commercial law and a case that reached the UK's equivalent of the Supreme Court have addressed the issue in limited contexts.
Most sources agree that COB is used to provide a degree of flexibility and means a different time in different contexts. COB depends on the factual and business circumstances, the context of the dispute, and the nature of the business. For example, if the business involves a banking dispute than the COB is likely the traditional 5:00 p.m. of the latest time zone in the dispute. However, if the dispute involves a commercial trucking company (whose hours of operation may extend to 7:00 p.m.) then the degree of flexibility is inherently different.
Additionally, treaties make it clear that close of business is not the same as end of the working day. In the US Code of Federal Regulation, it is determined, at least in one context, that “filed” means the complete receipt of any document by an agency before its close of business. Documents received after close of business are considered filed as of the next day. Unless otherwise stated, the agency close of business is presumed to be 4:30 p.m., local time. 48 C.F.R. § 33.101.
In the final analysis, COB should be what you define it to be, either in correspondence or contract. The best way to avoid a protracted and costly fight over the meaning is to simply define what it is so there is no doubt.