Useful for deadlines like "within 30 days of arrival" or "60 days before expiry".
Calendar days simply move the date by N days. Business days skip Saturdays and Sundays; public holidays are not known to the tool, so subtract them yourself. The start date itself is day 0, so adding 30 days to 1 March gives 31 March.
Does it count the start date? No. The start date is day 0, so day 1 is the next day.
Are public holidays excluded from business days? No, only Saturdays and Sundays are skipped. Remove holidays by hand if your deadline excludes them.
Which time zone is used? Dates are treated as plain calendar dates, so there is no time zone shift.
Informational tool, not legal or tax advice. Confirm requirements with the official authority. Everything runs in your browser; nothing is uploaded.