How do I add days to a date to find a deadline?

Enter your start date (e.g., today's date), select 'Add,' type the number of days, and choose 'Days' as the unit — the calculator instantly displays the resulting deadline date. This is ideal for counting 30-, 60-, or 90-day periods for invoices, trials, or legal notices.

What is 30 days from today?

The answer depends on today's date, which is why a calculator is more reliable than mental math. Simply enter today's date, add 30 days, and the tool accounts for month lengths and year boundaries automatically.

Does adding months always land on the same day of the month?

Usually yes — adding 3 months to March 15th gives June 15th. The exception is end-of-month dates: adding 1 month to January 31st gives February 28th (or 29th in a leap year) because that month is shorter, a behavior called end-of-month clamping.

How do I subtract days from a date to find a past date?

Select 'Subtract,' enter your reference date, input the number of days, and the calculator returns the earlier date. This is useful for finding out when something started if you know it ran for a set number of days.

Does this calculator include leap years?

Yes. The calculator correctly handles leap years, so adding 1 year to February 28, 2024 gives February 28, 2025, and adding 366 days to February 29, 2024 correctly lands on February 28, 2025.

What is the difference between adding days and adding months?

Adding days counts a fixed number of calendar days regardless of month boundaries. Adding months advances the month counter by the specified number while keeping the same day of the month (with end-of-month clamping). For a 30-day invoice term, use days; for a monthly subscription renewal, use months.

Can I use this to calculate a pregnancy due date?

Yes — a standard pregnancy is approximately 280 days (40 weeks) from the last menstrual period. Enter the first day of your last period, add 280 days (or 40 weeks), and the result is your estimated due date, matching the standard Naegele's Rule calculation.

Why might my manual date calculation be wrong?

Manual date math fails most often because people forget that months have different lengths, miscalculate whether a year is a leap year, or confuse whether the start date counts as Day 0 or Day 1. Using a date add/subtract calculator eliminates all three sources of error instantly.

Date Add / Subtract

Add or subtract days, weeks and months from any date to find a new date.

Compare

Start date

Months

Weeks

Days

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Resulting date

Wednesday, January 27, 2027

90 days after the start date

Day of week

Wednesday

ISO date

2027-01-27

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How the Date Add / Subtract works

The Date Add / Subtract calculator lets you instantly find a new date by adding or subtracting any number of days, weeks, or months from a starting date — perfect for planning deadlines, tracking project timelines, calculating due dates, or figuring out when a contract or subscription period ends.

At its core, this tool performs date math: it takes a starting date and offsets it forward or backward by a specified duration. Because dates aren't simple numbers — they account for months of varying lengths (28 to 31 days), leap years with an extra February 29th, and calendar boundaries — straightforward arithmetic would produce wrong answers. The calculator handles all of this complexity automatically, correctly rolling over month and year boundaries so you never end up with nonsense like 'March 32nd.'

When you add months rather than days, the calculation follows calendar-month logic rather than fixed-day-count logic. Adding one month to January 31st, for example, lands on February 28th (or 29th in a leap year) because February simply doesn't have 31 days. This 'end-of-month clamping' behavior is standard and matches how legal contracts, loan agreements, and subscription services typically interpret monthly intervals. If precision matters for your use case, verify whether your agreement specifies calendar months or a fixed number of days (e.g., 30 days vs. 1 month).

Leap years add another layer to date math that trips people up. A year is a leap year if it is divisible by 4, except century years must also be divisible by 400 (so 2000 was a leap year, but 1900 was not). This means adding 365 days to February 29th of a leap year does not land back on February 29th — it lands on February 28th the following year. The calculator resolves this correctly, but it is worth understanding if you are calculating recurring annual events tied to that rare date.

A common mistake when using date add or subtract tools is confusing calendar days with business days. This calculator counts every calendar day by default — weekends and public holidays included. If your deadline or contract term is measured in business days (common in legal filings, banking, and government processing), you will need a dedicated business-days calculator instead. For most everyday planning — pregnancy due dates, medication course end dates, warranty expirations, project milestones — calendar-day counting is exactly what you need.

Formula

New Date = Start Date + (N × Days per Unit)
where Unit = 1 day, 7 days (week), or calendar month;
for months: apply end-of-month clamping if the result day exceeds the target month's last day.

Pro tips

Use 'subtract days' to reverse-engineer a start date: if you know a deadline is in 90 days and want to find when the clock started, subtract 90 days from the deadline instead of adding from today.
When calculating legal or contractual deadlines, confirm whether the agreement counts the start date itself as Day 0 or Day 1 — this single-day difference can matter for court filings, insurance claims, and lease agreements.
For recurring events (monthly bills, quarterly reviews), add months rather than a fixed number of days so the result always falls on the same calendar date rather than drifting due to month-length variation.
Double-check any result that falls near February 28–29 or month-end dates; these are the most common places where manual date math goes wrong, and verifying the calculator output takes only seconds.
When planning project timelines, add your calculated end date to a calendar app immediately — the value of date math is zero if the result isn't anchored somewhere you'll actually see it before the deadline arrives.

Key terms

Date Math: — The process of adding or subtracting a duration from a calendar date while correctly handling month lengths, year boundaries, and leap years.
End-of-Month Clamping: — When adding months results in a day that doesn't exist in the target month, the date is adjusted down to the last valid day of that month (e.g., Jan 31 + 1 month = Feb 28).
Leap Year: — A year with 366 days (February has 29 days), occurring in years divisible by 4, except century years must be divisible by 400.
Calendar Days: — A count of every day including weekends and public holidays, as opposed to business days which exclude non-working days.
Duration Offset: — The number of days, weeks, or months you are adding to or subtracting from the start date to reach the target date.
Deadline: — A specific future date by which a task, payment, filing, or event must be completed, often calculated by adding a fixed interval to a known start date.