To add days to a date manually, start from your chosen date and count forward through the calendar, taking into account the varying lengths of each month and whether the year is a leap year.

1. Enter your start date in the date picker above
2. Type the number of days (or weeks, months, years) you want to add
3. Select Days as the unit from the dropdown
4. Click Add to calculate the future date
5. The result shows the full date, day of the week, and ISO format

Example: Adding 45 days to January 20, 2026 means counting 11 remaining days in January, 28 days in February (2026 is not a leap year), and 6 more days into March — giving March 6, 2026 (a Friday).

Adding months to a date is more complex than adding days because months have different lengths. The most common edge case is month overflow: adding one month to January 31 would land on February 31, which does not exist. JavaScript's built-in Date object handles this by rolling over into the next month — so January 31 + 1 month becomes March 3 (or March 2 in a leap year).

If you need to always land on the last day of the resulting month (e.g., Jan 31 + 1 month = Feb 28), you would need a custom clamping function. This calculator uses JavaScript's native overflow behavior, which is standard across date libraries.

A leap year has 366 days with February containing 29 days instead of 28. When you add days, weeks, or years to a date, the calculator automatically accounts for leap years so you always get the correct result.

• A year is a leap year if it is divisible by 4 (e.g., 2024, 2028)
• Century years (divisible by 100) are not leap years, e.g., 1900, 2100
• Exception: years divisible by 400 are leap years, e.g., 2000, 2400

Leap year example: Adding 1 year to February 29, 2024 (a leap day) would normally land on February 29, 2025 — but 2025 is not a leap year, so JavaScript rolls over to March 1, 2025.

The calculator uses JavaScript's native Date object for all arithmetic:

• Days: Calls setDate(getDate() + n), which automatically rolls over month and year boundaries
• Weeks: Multiplies the week count by 7 and uses the same day offset logic
• Months: Calls setMonth(getMonth() + n), with native overflow handling for short months
• Years: Calls setFullYear(getFullYear() + n), with leap day overflow for Feb 29

The "days from start" figure is calculated by dividing the millisecond difference between the result date and start date by 86,400,000 (milliseconds per day).