Date plus days
Adding a fixed number of days is unambiguous arithmetic. No month lengths, no clamping. A hundred days from Thursday 1 January 2026 is Saturday 11 April 2026.
How to add a day count
Fixed day counts turn up wherever a period must be unambiguous. Ninety-day payment terms, thirty-day returns windows and hundred-day political milestones are all day counts precisely because they cannot be argued about; a calendar month depends on which month, and a fixed count does not. The trade-off is drift, and it is predictable enough to use. Because a week is seven days, the weekday shifts by the remainder after dividing by seven: 7 and 14 land on the same weekday, 30 moves forward two, 90 moves forward six and so lands one weekday earlier, and 100 moves forward two. That is why a quarterly 90-day billing cycle creeps backwards through the week and eventually falls at a weekend. Adding 365 is the one case that looks safe and is not: it returns the same date only if no 29 February falls inside the span. From 1 March 2027, 365 days is 29 February 2028 rather than 1 March.
Questions
Enter 100 and read the result. The weekday is shown beside it.
Because a day count is unambiguous. A month depends on which month it is, and month-end starts have to clamp.
1 April in a common year, 31 March in a leap year.
The start weekday shifted forward by the day count divided by seven, remainder only. Ninety days shifts six, which is the same as one day earlier.
Only when no 29 February falls in between. Otherwise you land a day early.
No. Adding 1 day gives tomorrow, not today.
Yes. This counts every day. Use the business-day tools when they should be skipped.