Date Calculator

How to work it out yourself

1. 1.Set the starting date. It defaults to today, so "30 days from today" needs nothing but the number.
2. 2.Choose whether to count forward or back, and whether every day counts or only Monday to Friday.
3. 3.Read both lines. A contract that says thirty days and a contract that says thirty business days are two weeks apart, and which one was meant is worth settling before the deadline rather than after.

The formula

1. Days countedamount * (unit_days + 7 * unit_weeks)
2. Calendar days a business count covers7 * whole_weeks + odd_days + crosses
3. Counting every daystart + sign * span
4. Counting Monday to Friday onlyanchor + sign * business_shift
5. Datecalendar_result * count_calendar + business_result * count_business
6. Difference between the two countsabs(calendar_result - business_result)
7. That span in weeksabs(result - start) / 7

Source: ISO 8601 — date and time representation, US Office of Personnel Management — federal holidays

Questions people actually ask

What is the difference between calendar days and business days?

Calendar days count every day including weekends; business days count Monday to Friday only. Every five business days is exactly seven calendar days, so the gap widens by two days for every working week. Thirty business days is 42 calendar days, six weeks rather than four and a bit.

Are public holidays excluded?

No, and no calculator can do it honestly without knowing which country, state and sometimes which industry you are in. Federal holidays alone differ from state holidays, and bank holidays differ from court deadlines. Count the business days here, then subtract the holidays that actually apply to your calendar.

What if the starting date is a weekend?

For a business-day count it moves to the next working day first, forward when you are counting forward and back when you are counting back, then counts from there. A calendar count starts exactly where you put it.

Does this handle leap years?

Yes, because it never works in months. Every date becomes a day number, the arithmetic happens on integers, and the number becomes a date again at the end. February has 29 days in 2028 and the count crosses it without a special case.

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