FDA deadlines mix both — a "working day" excludes weekends and federal holidays, a "calendar day" doesn't.
21 CFR 803.3 (calendar day); 21 CFR 806.10 (working day)
What people get wrong
Getting this wrong is a purely avoidable way to blow a deadline. The MDR reporting window, for example, runs in calendar days, while some correction-and-removal reporting windows run in working days — the same numeric deadline ("10 days") lands on different actual dates depending on which one applies. Always confirm which one a specific regulation uses rather than assuming; don't let a long weekend quietly eat two of your working days.
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