30 Day Calculator

About the 30 Day Calculator

What does a 30 day calculator do?

A 30 day calculator finds the date exactly 30 days before or after a start date.

The count uses calendar days on the Gregorian calendar, not months, so the answer depends on whether the months it crosses have 28, 29, 30 or 31 days. In practice it is a 30 day calendar calculator. It can also count 30 business days or the days between two dates, and today's date comes from your own device clock.

What are the key facts about a 30-day period?

A 30-day period is 4 weeks and 2 days long.

• A Friday plus 30 days is a Sunday. Twenty-eight of the 30 days are whole weeks, so only the last two move the weekday.
• Four months have exactly 30 days. They are April, June, September and November.
• A calendar month averages 30.44 days. The Gregorian year of 365.2425 days divided by 12 gives 30.436875.
• Six weeks pass before the 30th business day. From any weekday start, it arrives 42 calendar days later on the same weekday.

How do you count 30 days from a date?

Enter the start date and keep 30 as the number of days.

1. Set the start date. It opens on today, and End of this month jumps to the last day of the current month.
2. Keep 30 days or pick 14, 45 or 90. Any whole number up to 100,000 works.
3. Pick After or Before. Before turns it into a days before calculator for the date 30 days ago.
4. Check the month comparison. The box under the result shows where one calendar month from the same start lands.

When does 30 days equal one month?

Thirty days reaches the same date next month only when you start in a 30-day month.

From any date in April, June, September or November, 30 days later is the same date next month. From the 1st through the 28th of a 31-day month, it lands one day short. From February it lands two days past the same date in March, or one day past it in a leap year.

Starting on the 1st, seven of the twelve months give a 30-day date one day before the month date. A 30-day trial that starts on July 1 ends on July 31, a day earlier than a one-month trial from the same date.

Why does January 31 plus one month cause trouble?

February has no 31st, so a month after January 31 has no exact match.

Thirty days after Saturday, January 31, 2026 is Monday, March 2, 2026, or Wednesday, March 1 in the leap year 2028. Spreadsheet month functions such as Excel's EDATE stop at the end of February instead, on February 28, 2026 or February 29, 2028.

JavaScript does something else again. new Date(2026, 0, 31) followed by setMonth(1) rolls over to March 3, 2026, because the spare days carry into the next month, as MDN explains for Date.prototype.setMonth(). The 30 day calculator counts days, so it gives the March 2 answer and shows the clamped month date beside it.

What does net 30 mean on an invoice?

Net 30 means the full amount is due 30 calendar days after the invoice date.

An invoice dated Friday, September 25, 2026 on net 30 terms is due Sunday, October 25, 2026. Net 30 EOM starts the 30 days at the end of the invoice month instead, so the same invoice would be due Friday, October 30, 2026.

Tap End of this month to count from the last day of the current month. When a due date lands on a weekend, the invoice terms decide whether it moves, so read them before you schedule the payment.

Common mistakes

These 30-day mistakes come from mixing up days and months.

• Writing one month when the rule says 30 days. From February 1, 2026 the two answers are two days apart, March 3 against March 1.
• Adding a month in code for a 30-day term. Month arithmetic clamps or overflows at month ends, so add the days directly with date.setDate(date.getDate() + 30) in JavaScript or =A2+30 in a spreadsheet.
• Reading 30 business days as 30 days. Thirty business days take 40 to 42 calendar days, so a deadline read the wrong way comes up to 12 days early.

Common Use Cases

A 30-day count shows up in trials, returns, invoices and notices.

• Free trials. Use it as a 30 day counter to find the last day before a trial turns into a paid plan.
• Return windows. Find the last day a store takes a return, counted from the purchase or delivery date.
• Invoices on net 30. Get the due date from the invoice date, or from the month end for net 30 EOM.
• Notices and lookbacks. Count forward for a 30-day notice, or back to find the date 30 days ago.

Shorter windows are easier to read with the 10 day calculator, which shows where the weekday lands, while the 60 day calculator handles two-month spans. The business days calculator counts the working days between two dates you already know.

Frequently Asked Questions

Is 30 days the same as one month?

No, 30 days reaches the same date next month only when you start in April, June, September or November. Calendar months run from 28 to 31 days, so from any other month the same day number does not come around after exactly 30 days.

Is a month 4 weeks or 30 days?

Neither exactly, since 4 weeks is 28 days and calendar months run from 28 to 31 days. The average month in the Gregorian calendar is 30.44 days.

What is net 30 days from today?

Net 30 from today is the date 30 calendar days after today, when an invoice dated today falls due. With net 30 EOM, the 30 days start from the last day of the current month.

How do you add 30 days to a date in JavaScript?

Call date.setDate(date.getDate() + 30), which rolls over month and year ends on its own. Avoid setMonth() for a 30-day term, because a month is not always 30 days.

How do you count 30 days without weekends?

Count 30 business days, Monday through Friday only, which takes six weeks from any weekday. Thirty business days from Friday, September 25, 2026 end on Friday, November 6, 2026.

How many days are in each month?

April, June, September and November have 30 days, and February has 28, or 29 in a leap year. The other seven months have 31 days.