Why another date calculator
Because most of them give you a number and no way to check it. Ask two calculators for "30 days from today" and they will usually agree; ask them for "30 business days" and they often will not, because one counts the start date, one uses a different holiday list, and neither says which. The disagreement is never in the arithmetic — it is in the convention, and the convention is what almost nobody prints.
So every answer here comes with its working: the day of the week, the ISO week, the business-day equivalent, which holidays fell inside the span, and whether the start date was counted. Where our answer legitimately differs from another tool's — 31 January plus one month is 28 February here, not 3 March — the page says so and says why.
How it works
Every calculation runs in your browser; nothing you type is sent anywhere. Each page also ships with the answer already computed, so it is correct and readable before any JavaScript runs and on any connection. Several hundred of these pages answer a question about "today", and those pages work their own answer out again in your browser for the date you open them, so the answer does not go stale between one visit and the next.
The full set of conventions is on the methodology page — the counting rule, month-end clamping, the leap-year rule, ISO versus US week numbering, the business-day rule and where the holiday dates come from. It is worth two minutes if you are counting a deadline.