Add 30 Days to Date Calculator
Calculate exactly 30 days from any starting date. This common calculation is essential for billing cycles, payment terms, subscription renewals, and legal deadlines. Our calculator handles month transitions and leap years automatically.
Common 30-Day Uses
| Use Case | Starting Point | Example |
|---|---|---|
| Net-30 invoices | Invoice date | Due date calculation |
| Trial periods | Signup date | Trial expiration |
| Return policies | Purchase date | Return deadline |
| Prescription refills | Last fill date | Refill date |
| Monthly billing | Bill date | Next payment |
Add 30 Days Calculator
``javascript
function add30Days(startDate) {
const start = new Date(startDate);
const result = new Date(start);
result.setDate(result.getDate() + 30);
return {
startDate: start.toLocaleDateString('en-US', {
weekday: 'long', year: 'numeric', month: 'long', day: 'numeric'
}),
resultDate: result.toLocaleDateString('en-US', {
weekday: 'long', year: 'numeric', month: 'long', day: 'numeric'
}),
dayOfWeek: result.toLocaleDateString('en-US', { weekday: 'long' }),
daysAdded: 30
};
}
`
30 Days vs. One Month
Note that 30 days is not the same as "one month." January 15 + 30 days = February 14, but January 15 + 1 month = February 15. For legal and financial purposes, "30 days" usually means exactly 30 days, while "one month" follows calendar months. Clarify which applies to your situation.
Business Days Consideration
If you need 30 business days (excluding weekends), the calendar result will be approximately 42 calendar days. Some legal and business contexts specify "business days"—always verify which calculation applies to your deadline.
The Traps in Date Arithmetic
Months are not 30 days. Adding one month to 31 January has no correct answer, and every
library picks differently — 28 February, 2 March or an error. Decide which your business
needs before the edge case decides for you.
Days are not always 86,400 seconds. Daylight saving makes one day 23 hours and another
25. Adding n * 86400000 milliseconds drifts by an hour twice a year, which is enough to
move a date across midnight and return an answer one day out.
`javascript
// Correct — the Date object normalises overflow and handles DST
const addDays = (date, days) => {
const result = new Date(date);
result.setDate(result.getDate() + days);
return result;
};
// Correct — calendar days, unaffected by DST or time of day
const daysBetween = (a, b) => {
const utc = (d) => Date.UTC(d.getFullYear(), d.getMonth(), d.getDate());
return Math.round((utc(b) - utc(a)) / 86400000);
};
`
Months are zero-indexed in JavaScript. new Date(2026, 0, 1) is January. new
Date(2026, 12, 1) is January 2027, silently.
Parsing is not portable. new Date("2026-08-30") is parsed as UTC, while
new Date("2026/08/30")` is parsed as local — the same calendar date, a day apart in some
zones. Always parse ISO 8601, and construct with explicit components when the time matters.
Time Zones Decide the Answer
| Question | Depends on |
|---|---|
| How many days until an event? | The viewer's zone |
| Is this subscription expired? | UTC, always |
| What day of the week was this? | The zone the event happened in |
| When does "tomorrow" start? | The viewer's zone |
Reference Points
| Interval | Days |
|---|---|
| Week | 7 |
| Month (average Gregorian) | 30.44 |
| Quarter | 91.31 |
| Year | 365.2425 |
| Leap year | 366 |