Day Counter Calculator

This day counter calculator counts days between two dates or adds and subtracts business days from a starting date. It is built for project managers tracking deadlines, HR professionals counting notice periods, legal teams calculating filing dates, and anyone who needs to know how many working days remain before a target date. The tool outputs total calendar days, weekday count, weekend count, and excluded holidays in a detailed breakdown. In add/subtract mode, it steps forward or backward by a specified number of business days, skipping weekends and observed holidays from the US, UK, Canada, or a custom list. It does not track hours or minutes, and it does not model partial days. The Doomsday Algorithm proof in the Information Hub shows the weekday math behind any date. When your endpoints are two birthdays and you want an age-style breakdown instead of a raw day count, pair this tool with the Age Difference Calculator for years, months, and days between those births.

Total days are calculated from the millisecond difference between two dates, divided by 86,400,000 (the number of milliseconds in a day). The inclusive or exclusive toggle determines whether the end date itself is counted. Weekday and weekend totals use a constant-time formula based on the starting day of the week and the total span—no per-day loop is needed, so results are instant even for wide ranges. Business-day stepping in add/subtract mode walks forward or backward through weekdays only, checking each candidate date against the holiday exclusion list. Holiday lists are generated dynamically for US Federal, UK (England and Wales), and Canada Federal calendars, or you can supply custom dates in YYYY-MM-DD format. The Doomsday Algorithm verifies the weekday of any date using modular arithmetic and a century anchor.

Total days come from the millisecond difference between the two dates (inclusive or exclusive end). Weekday and weekend counts use a constant-time formula from the start date's day-of-week. The calculator's Information Hub above explains Duration mode, Add/Subtract mode, and holiday exclusions in one place.

Calendar days include every day, useful for lease terms, visa validity, or "X days from today." Business days exclude weekends and optionally holidays, the norm for project timelines and notice periods. Use Duration Between for the full breakdown; use Add/Subtract with "Count Business Days Only" to step through weekdays only. The Information Hub above summarizes how each mode works.

The Doomsday Algorithm finds the weekday of any date using a century anchor and a short formula on the year. The calculator's Information Hub includes a Doomsday Rule tile with the formula and a Doomsday Proof tile that runs through the steps for your selected start date.