Date Calculator (Add/Subtract)

The core of date arithmetic relies on Unix timestamp arithmetic. Every date can be represented as the number of milliseconds elapsed since the Unix epoch (January 1, 1970, 00:00:00 UTC). By converting dates to this numeric representation, addition and subtraction become straightforward integer operations.

The fundamental formula for adding days is:

$$T_{result} = T_{start} + (d \times 86{,}400{,}000)$$

where $$T_{result}$$ is the result timestamp in milliseconds, $$T_{start}$$ is the start date timestamp in milliseconds, and $$d$$ is the number of days to add. The constant $$86{,}400{,}000$$ is the number of milliseconds in one day ($$60 \times 60 \times 24 \times 1000$$).

For subtraction:

$$T_{result} = T_{start} - (d \times 86{,}400{,}000)$$

After computing $$T_{result}$$, the value is converted back to a calendar date by extracting the year, month, and day components. The day of the week is determined using the modular relationship:

$$w = \left(\left\lfloor \frac{T_{result}}{86{,}400{,}000} \right\rfloor + 4 \right) \mod 7$$

where $$w = 0$$ corresponds to Sunday, $$w = 1$$ to Monday, through $$w = 6$$ to Saturday. This approach automatically handles leap years and month-length variations because the Gregorian calendar rules are embedded in the JavaScript Date object's UTC conversion logic.

The algorithm correctly handles edge cases such as crossing from December 31 to January 1 (year increment), February date arithmetic in leap years (adding 1 day to February 28 in a leap year gives February 29, not March 1), and subtraction that goes back across year boundaries.

The Result Date output shows the exact calendar date obtained after adding or subtracting the specified number of days from your start date, formatted as YYYY-MM-DD for unambiguous international readability. The Day of Week output tells you the weekday name of the result date — helpful for scheduling since you may want to avoid weekends or target a specific business day.

If you are calculating deadlines, note that legal and business conventions sometimes specify "business days" (excluding weekends and public holidays) rather than calendar days. This calculator uses calendar days. If your result falls on a Saturday or Sunday, consider adjusting to the nearest weekday as required by your context. For recurring deadlines (e.g., monthly invoices), use the Add Months calculator instead, as calendar months vary in length.