Add Days to Date
Find out what date it will be after adding or subtracting days.
Why day-based date arithmetic matters
Adding "one month" to January 31 is ambiguous — does it mean February 28, March 2, or March 3 (in a leap year)? Day-based arithmetic is completely unambiguous: adding 30 days to January 31 always gives March 2 (or March 1 in a leap year). For contracts, deadlines, and legal timelines, days are the precise and legally safe unit.
Common date-addition scenarios
| Scenario | Days to add | Business context |
|---|---|---|
| Return window | +14 days | Standard consumer return policy (EU: 14 days mandatory minimum) |
| Net-30 payment | +30 days | Invoice due date for standard B2B payment terms |
| Net-60 / Net-90 | +60 / +90 days | Extended B2B payment terms for larger transactions |
| Trial expiry | +7 or +30 days | Free trial period end date for software subscriptions |
| Quarterly deadline | +90 days | Tax filing, financial reporting, and board meeting cycles |
| Half-year | +183 days | Semi-annual reporting or review periods |
| Annual renewal | +365 days | Subscription, domain, or certification renewal date |
| Probation end | +90 days | Standard employment probation period in most countries |
How date addition works technically
JavaScript's Date object stores time as milliseconds since January 1, 1970 (Unix epoch). Adding days means adding that many multiples of 86,400,000 milliseconds (24 hours × 60 minutes × 60 seconds × 1,000 ms). This automatically handles month boundaries, different month lengths, and leap year February.
1 day = 24h × 60m × 60s × 1000ms = 86,400,000 ms
Add 30 days to January 31:
Jan 31 timestamp + (30 × 86,400,000) = March 2 timestamp
Add 30 days to January 31 in a leap year (2024):
Jan 31, 2024 + 30 days = March 1, 2024 (Feb has 29 days in 2024)Subtracting days: working backwards
Use a negative number to subtract days and calculate past dates. This is useful for determining when a deadline started ("the contract was signed 90 days before today"), when a subscription was first activated, or when a legal countdown began. Enter a negative value in the input field above to calculate backwards.
Business days vs. calendar days
This calculator adds calendar days (all 7 days of the week). Many business deadlines specify "business days" or "working days" — typically Monday through Friday, excluding public holidays. For a 10 business-day deadline, the actual calendar days span depends on how many weekends fall within the period (usually 14 calendar days for 10 business days).
Frequently asked questions
What does the add days calculator do?
It adds or subtracts a number of days from any start date and returns the resulting calendar date, accounting for month lengths and leap years.
Can I subtract days as well as add them?
Yes. Enter a negative number of days, or use the subtract option, to find a date in the past.
Does it count weekends and holidays?
It counts every calendar day, including weekends and holidays. For working-day calculations you would exclude those separately.
How does it handle leap years?
It uses the real calendar, so February 29 in leap years is included automatically when counting across that date.
Calculates what the date will be after adding or subtracting a number of days from a starting date. Quick preset buttons for common intervals.
result = start + (days × 86,400,000 ms). Negative values subtract days.