Date Calculator

Date Math, Made Easy: Days Between, Adding/Subtracting, Working Days, ISO Week Numbers, and Day‑of‑Year

1) What a date calculator can do

This all‑in‑one date calculator covers everyday tasks you’d otherwise do with a wall calendar and a lot of patience: measure the distance between two dates, add or subtract days, months, or years, count working days while skipping weekends and holidays, and look up calendar facts for any date—ISO week number, day of year, and fiscal quarter. All logic is transparent and based on calendar days, so results are easy to verify in any planner or spreadsheet.

2) Days between two dates (exclusive vs inclusive)

There are two sensible ways to measure the span between dates. The default is exclusive: it counts how many midnights you pass going from the start date to the end date. For example, 1 Jan to 2 Jan is 1 day. The inclusive mode adds 1 day so that a single‑day event shows as 1 day long. Both are useful; the toggle in this app lets you choose.

3) Exact spans in years, months, and days

Years and months are not fixed lengths, so you can’t just divide days by 30. This tool computes the total number of whole months between the two dates, borrowing a month if the ending day of the month hasn’t been reached. From those months we derive years and months, and the leftover days are calculated from an anchor date. This matches how people talk about age or seniority—“2 years, 3 months, and 12 days.”

4) Adding and subtracting years, months, and days

When adding months to a date like Jan 31, February may not have a 31st. Our calculator “snaps” to the last valid day of the target month (Feb 29 in leap years, otherwise Feb 28). Offsets accept positive or negative numbers, so subtracting is as simple as entering negatives. You can also add a number of business days, which automatically skips weekends and optional holidays you list.

5) Business days, weekends, and holidays

Business days are Monday through Friday by default. Real calendars also include public holidays, which you can paste line‑by‑line. Once set, the tool can count business days between two dates or add a business‑day offset to a base date to find the next target date for project schedules, SLAs, and delivery promises.

6) ISO week numbers and why they matter

Many teams and countries schedule work by ISO‑8601 weeks. Weeks start on Monday, and week 01 is the week with the year’s first Thursday (equivalently, the week containing 4 January). The standard avoids short “week 0/53” edge cases around New Year. This app displays the ISO week number for any date and the Monday–Sunday range for that week.

7) Day of year, days left in year, and quarters

The day of year (DOY) is handy for quick comparisons and for logging systems. It runs from 1 to 365 (or 366 in leap years). We also show how many days remain in the year, which quarter the date belongs to, and the first/last day of the month and quarter.

8) Common pitfalls and pro tips

• Time zones: This tool uses your local calendar and ignores time‑of‑day, so daylight‑saving changes do not affect results.
• Inclusive vs exclusive: Be clear which convention you need—contracts and HR often use inclusive counting.
• Holiday lists: Keep a copy of your organization’s official holidays and paste them into the holidays box to standardize results.
• Month‑end additions: Expect snapping to the end of the month for dates like the 29th–31st.

9) How to use this tool step‑by‑step

1. Pick the tool: Difference, Add/Subtract, Business Days, or Week & DOY.
2. Enter the required dates (and optional holiday list).
3. Press Calculate to see key results up top and detailed breakdowns below.
4. Use Copy Share Link to save your inputs, or Download / Print to keep a record.

10) FAQ

This calculator uses deterministic calendar arithmetic and is intended for general planning and education.