How this calculator works
A date is converted to its day number in the proleptic Gregorian calendar, the offset is added, and the day number is converted back to a date. Every calendar day counts, so weekends, statutory holidays and February 29 are all ordinary steps. Because the arithmetic stays on whole days, no time zone or daylight-saving rule can add or remove a day.
Formula
Let O(d) be the day number of date d, with 0001-01-01 as day 1. The result has day number O(start_date) + offset_days. Its ISO weekday is 1 + ((day number - 1) mod 7), where 1 is Monday and 7 is Sunday.
Worked example
Two days after 2024-02-28 is 2024-03-01, a Friday. One day before 2025-01-01 is 2024-12-31, a Tuesday. Starting at 9999-12-31 and adding one day leaves the supported range.
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Frequently asked questions
Does this account for leap years?
Yes. The leap day is counted like any other day, so adding 365 days to a date before a leap day lands one day earlier than adding a full year would.
Can I subtract days as well as add them?
Yes. Enter a negative offset to move backward. A zero offset returns the starting date and its weekday unchanged.
Why is the weekday shown as a number?
The ISO weekday number avoids ambiguous abbreviations across languages: 1 is Monday, 5 is Friday, 7 is Sunday.