Date Calculator
Add or subtract days, weeks, months, or years from any date.
Whether you are counting forward to a contract deadline, calculating a notice period, working out when a visa expires, estimating a pregnancy due date, or simply trying to figure out what date falls 90 days from now, a date calculator is an essential everyday tool. Manually adding days to a date is tedious and error-prone once you factor in months of different lengths, leap years, and year boundaries. A seemingly simple question like "what is 60 days from March 15?" becomes surprisingly tricky when February is involved.
Our free date calculator lets you add or subtract any number of days, weeks, months, or years from any starting date and instantly see the result. The output includes the calculated date, the day of the week, the ISO week number, and how many days the result is from today. Quick-preset buttons let you jump to common offsets like +7 days, +30 days, +90 days, or +1 year with a single click. You can also copy the result or share a direct link with the calculation pre-filled. Everything runs in your browser with no data sent to any server.
How to Add or Subtract Days from a Date
Adding or subtracting time from a date is one of the most common calculations people need. From figuring out a 30-day return window to calculating when a 6-month probation period ends, knowing the exact future or past date saves you from calendar-flipping and miscounting. The challenge is that months have different lengths (28, 29, 30, or 31 days), leap years add an extra day in February, and year boundaries can make mental arithmetic unreliable. Our date calculator handles all of these edge cases automatically.
How to Use This Tool
- Set your start date — the date field defaults to today. Click or tap to choose a different starting date if needed.
- Choose Add or Subtract — use the mode dropdown to select whether you want to move forward or backward in time from your start date.
- Enter a number — type the quantity you want to add or subtract. For example, enter 90 if you need the date 90 days from now.
- Select the unit — choose Days, Weeks, Months, or Years from the unit dropdown to specify what the number represents.
- Click Calculate — the result appears instantly showing the calculated date, day of the week, ISO week number, and how many days the result is from today. Alternatively, use one of the quick-preset buttons for common calculations.
How It Works Behind the Scenes
The calculator uses JavaScript's Date object to perform date arithmetic. When you add or subtract days, the tool calls setDate(getDate() + n) which correctly rolls over month and year boundaries. For weeks, it multiplies by 7 and adds days. For months, it uses setMonth(getMonth() + n), which handles varying month lengths — if the resulting day does not exist in the target month (for example, adding 1 month to January 31), JavaScript automatically rolls to the last valid day. For years, it uses setFullYear(getFullYear() + n) with the same overflow logic. The ISO week number is calculated using the standard algorithm that counts Thursday-based weeks from January 1. All computation runs entirely in your browser.
Frequently Asked Questions
What is 30 days from today?
Open the date calculator with the start date set to today (the default), ensure the mode is set to Add, type 30 in the number field, select Days, and click Calculate. The result shows the exact date 30 days from now along with the day of the week. You can also click the "+30d" preset button for an instant answer. This is commonly needed for payment terms, return policies, and trial periods.
What is 90 days from today?
Use the "+90d" preset button or enter 90 days manually. The calculator shows you the exact date 90 days from today, which is often needed for probation periods, visa validity windows, quarterly deadlines, and 90-day fitness or health challenges. The result includes the day of the week so you can see whether it falls on a weekday or weekend.
How does adding months work when months have different lengths?
When you add months, the calculator moves to the same day-of-month in the target month. If that day does not exist (for example, adding 1 month to January 31 targets February 31, which does not exist), the calculator rolls forward to the next valid date — in this case March 3 (or March 2 in a leap year). This follows JavaScript's standard Date behaviour and matches how most legal and financial systems handle month arithmetic.
Can I subtract days from a date to find a past date?
Yes. Switch the mode from Add to Subtract using the dropdown, enter the number of days (or weeks, months, years), and click Calculate. This is useful for working backwards from a deadline to find a start date, determining when a warranty or guarantee period began, or calculating historical dates relative to a known event.
Does the calculator handle leap years correctly?
Yes. The calculator uses JavaScript's Date object which correctly implements the Gregorian calendar leap year rules: a year is a leap year if it is divisible by 4, unless it is divisible by 100, unless it is also divisible by 400. This means calculations crossing February 29 in a leap year are handled accurately, whether you are adding days, weeks, months, or years.
Can I use this for contract and deadline calculations?
Absolutely. The date calculator is designed for exactly this kind of use. Enter the contract start date, add the notice period or contract duration, and instantly see the end date. Common scenarios include 30-day notice periods, 90-day probation periods, 6-month tenancy agreements, 1-year warranty expiry dates, and multi-year contract renewals. The result includes the day of the week so you can check whether the deadline falls on a working day.
