Real holiday calendars and per-country weekends, the same data the API serves. Pick a region, pick a calculation, get a date you can rely on.
Where naive business-day math goes wrong
| You compute | Reality | Off by |
|---|---|---|
| +3 business days from Thu Dec 23, 2027 (UK), skipping only Sat/Sun → Tue Dec 28 | Dec 25 and 26 fall on the weekend, so the UK observes the substitute holidays Mon 27 and Tue 28. Neither counts as a working day. | 2 days |
| +5 business days from Sun Jan 3, 2027 (Saudi Arabia), assuming a Mon-Fri week → Fri Jan 8 | Saudi Arabia's weekend is Friday and Saturday, not Saturday and Sunday. Sunday is a working day and Friday isn't. | 2 days |
| Business days from Dec 20, 2027 to Jan 3, 2028 (UK) estimated as days x 5/7 → 11 | The window holds two weekends plus Christmas, Boxing Day, and New Year's Day (with substitutes). The real count is 8. | 3 days |
All three use this API's own data for the stated region and year. Ratios and weekday-only logic break the moment a holiday or a non-Saturday/Sunday weekend enters the window, which is most of the time outside the US.
Use it in your code
curl "https://dierum.io/v1/add?region=US&start=2027-01-15&days=10" \
-H "X-Api-Key: $KEY"
curl "https://dierum.io/v1/count?region=US&from=2027-01-01&to=2027-01-31" \
-H "X-Api-Key: $KEY"Get a free key, 100 requests a day, no card. The docs carry the same two calls in Python, JavaScript, and C#, and the playground covers due dates, paydays, and bridge days without writing any code.
Notes
Weekends vary by country (Friday-Saturday across much of the Middle East, Saturday only in Nepal). Substitute holidays are included: when 2027-12-25 falls on a Saturday, the UK calendar carries the observed days that follow. Market calendars (XNYS, ECB) differ from national ones.
More calculators
Invoice due datesPayroll datesHoliday calendarsCommon working daysSettlement dates (T+1/T+2)SLA deadlines
Country calculators
Each page carries that country's upcoming holidays and weekend rule.