Why Your Business Day Count Was Off by One Day
The most common cause is inconsistency about whether the start date counts as day one — counting it in one calculation and excluding it in another produces exactly a one-day discrepancy, which is the single most frequent source of manual business-day counting errors.
An off-by-one error is frustrating precisely because the math otherwise feels obviously correct — it almost always comes down to one small, specific assumption made differently between two counts.
The core ambiguity
"10 business days from Monday" can mean either "the 10th weekday after Monday, not counting Monday itself" or "Monday plus 9 more weekdays, counting Monday as day one" — both are reasonable readings of the same instruction, and they produce results one business day apart.
How to eliminate the ambiguity
The reliable fix is being explicit up front: decide whether the start date counts, write that convention down alongside any deadline calculation, and apply it consistently. When working with a contract, legal notice, or anything with real consequences, check the specific document's own definition rather than assuming a default.