Calendar Days vs. Business Days Between Two Dates
A calendar-day count includes every day, weekends included. A business-day count excludes Saturdays and Sundays, so it's always smaller (or equal, for a span with no weekend) than the calendar-day count for the same two dates — the gap grows by roughly 2 days for every 7 calendar days in the span.
This is the same core distinction covered in the business-days guide, applied specifically to comparing a total day count between two fixed dates rather than projecting forward from a start date.
How much the two counts typically diverge
A 30-calendar-day span typically contains around 8–9 weekend days, making the equivalent business-day count roughly 21–22 — a meaningful difference that grows larger for longer spans, since each additional week adds another 2-day gap.
When to use which count
Calendar days are the right measure for anything tied to actual elapsed time regardless of business operations — age, a billing cycle, a subscription period. Business days are the right measure for anything tied to when institutions actually operate — bank processing, legal notice periods, most contractual deadlines.
Checking which one a specific situation calls for
When it's not obvious which count applies, checking the specific document, policy, or context that defines the timeframe (rather than assuming) avoids a real, sometimes consequential misunderstanding, given how large the gap between the two counts can be.