Date addition and subtraction: days are not months
This tool adds or subtracts a duration from a starting calendar date. The amount can be entered as days or weeks; one week is treated as seven days. It’s useful for schedules, project planning, return dates, and checking a date a fixed number of days away.
Worked example
Adding 30 days to January 10 does not mean “one calendar month later.” The result depends on the number of days in the month the calculation crosses. February and leap years are common reasons that mental estimates differ.
This tool does not know the rules for a legal deadline, a contract, a business day, or an official filing period. Those may exclude weekends or holidays, count the first day, or use a jurisdiction-specific rule. For those tasks, confirm whether the rule means calendar days or business days and whether the count is inclusive.
Tips for accurate results
Choose whether you are counting calendar days, weeks, months, or years, because they behave differently. Adding a month to January 31 has no exact equivalent in a shorter month, so tools typically land on the last day of that month. For business deadlines, remember that weekends and holidays are not excluded unless you specifically account for them.
Common mistakes and limits
Leap years add February 29, which shifts results by a day when your range crosses one. Contracts and legal deadlines may define how to count, such as whether the start day is included, so check the wording. This calculator uses calendar dates and does not account for time zones.