January 1, 2026 to December 31, 2026 — a full calendar year — comes out to 364 days between the two dates (not 365, since the count is the gap between the dates rather than the number of days the year contains), which breaks down evenly to 52 weeks and 0 days. Of those 364 days, 260 fall on a weekday. That last number is genuinely useful on its own — it's the reason a "10 business day" shipping estimate or a project deadline quoted in working days takes roughly two calendar weeks to cover every 10 weekday count, since weekends eat almost 30% of any date range.
Starting from September 18, 2026 and adding 90 days lands on December 17, 2026, a Thursday — correctly rolling through the different lengths of September, October, and November along the way without any manual adjustment. That's the entire value of doing this with a calculator instead of by hand: manually adding 90 days means separately tracking how many days are left in the current month, then the next, then whatever's left over, and getting the day-of-week right requires counting through all of it. Entering a negative number in "Add / Subtract Days" runs the same logic backward from the start date.
The total days figure includes every day. A separate weekdays row shows the Monday–Friday count if you need business days specifically.
Yes — in "Add / Subtract Days" mode, enter a negative number to go backward from your start date.
Yes — all date math is calculated using actual calendar dates, so leap years are handled automatically.