Free Working Days Calculator

How It Works

This tool runs entirely in the browser with no external libraries — all date arithmetic is hand-written JavaScript using the native Date object. It has two modes: counting working days between two dates, and projecting a future (or past) date by adding or subtracting a given number of business days.

1. 1
   Date parsing without UTC drift

   Date inputs are parsed by splitting the YYYY-MM-DD string into year, month, and day parts and constructing a local Date — not Date.parse() — so the result is always the calendar day you typed, regardless of your timezone offset.
2. 2
   Holiday validation and lookup

   Each holiday entry is matched against a strict YYYY-MM-DD regex, then cross-checked by reconstructing the Date and comparing its parts back — so impossible dates like 2026-02-31 are rejected and flagged in the UI rather than silently rolling over to March. Valid entries are stored in a Set keyed by local date string for O(1) lookup during the loop.
3. 3
   Day-by-day iteration with configurable weekends

   For the 'count between dates' mode, the tool walks every calendar day in the range, checking each against your configured weekend days (any combination of the seven days) and the holiday Set, then tallies working days, weekend days, and holidays hit separately. For 'add business days' mode, it steps forward or backward one day at a time, skipping non-working days, until the requested count of business days has been consumed.
4. 4
   Locale-aware result display

   The final date in 'add' mode is formatted with the browser's built-in Intl via toLocaleDateString(), so it renders in the user's local language and calendar convention without any additional library.

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