Overview
What this calculator does
The date duration calculator does two things. It counts the number of days, weeks, months, and years between two dates (including business days vs weekend days). And it adds or subtracts any combination of years, months, weeks, and days from a starting date to find a future or past date. All of it runs in your browser using UTC date math to avoid daylight-saving bugs.
Counting duration
Days between two dates
The basic count is the later date minus the earlier date. The subtlety is whether to include the end date:
- Exclusive (default): June 1 to June 8 = 7 days. This is the gap, like counting nights in a hotel.
- Inclusive: June 1 to June 8 = 8 days. This counts both endpoints, like counting days an event runs.
The calculator also breaks the duration into a calendar-aware years/months/days format, so 400 days becomes "1 year, 1 month, 5 days" rather than a raw number.
Working time
Business days vs weekend days
The calculator separates business days (Monday through Friday) from weekend days (Saturday and Sunday). This is useful for:
Project deadlines
A '10 business day' turnaround spans two calendar weeks. Knowing the weekend split tells you the real calendar date.
Shipping estimates
Couriers quote business days. A 5-business-day shipment ordered Thursday arrives the following Thursday, not Tuesday.
Payment terms
'Net 30' usually means calendar days, but some contracts use business days. The distinction matters near month-end.
Notice periods
Employment and legal notice periods sometimes count only working days. Verify which your contract uses.
Holidays aren't counted
Finding a target date
Adding and subtracting from a date
Set a starting date, choose add or subtract, and enter any combination of years, months, weeks, and days. Common uses:
- Return windows: 'purchased + 30 days' for a refund deadline
- Contract terms: 'signed + 1 year' for a renewal date
- Visa durations: 'entry + 90 days' for a stay limit
- Project milestones: 'kickoff + 6 weeks' for a checkpoint
- Medical: 'last dose + 14 days' for a follow-up
Month and year math follows calendar rules: adding 1 month to January 31 gives February 28 (or 29 in a leap year), since there's no February 31.
Avoiding the DST trap
Why UTC?
Date-only calculations in local time can break around daylight saving transitions, where a "day" might be 23 or 25 hours. Computing in UTC (where every day is exactly 24 hours) avoids off-by-one errors. Since this tool works with dates, not specific times of day, UTC gives clean, correct results.
Behind the scenes
Privacy and how it runs
Runs in your browser
Common questions
How many days are between two dates?
Subtract the earlier from the later. The calculator does this and also shows weeks, an average month count, years, and a precise years/months/days breakdown.
How do I count days until a future event?
Set today as the start and the event as the end date. The total days is your countdown. Popular for weddings, exams, vacations, and product launches.
What's the difference between calendar days and business days?
Calendar days count every day. Business days count only Monday through Friday. A 10-business-day period spans 14 calendar days (two weekends). Always confirm which a contract or service means.
How do I add 90 days to a date?
Use the add/subtract tab, set days to 90, choose 'Add'. The tool returns the exact date and day of the week. Common for return windows and visa stays.
Does the calculator handle leap years?
Yes. Leap years (Feb 29) are handled automatically in both the day count and the add/subtract math. Adding a year to Feb 29, 2024 gives Feb 28, 2025, since 2025 isn't a leap year.
Why might my count be off by one?
Almost always the inclusive vs exclusive end date. Toggle the 'include the end date' checkbox to switch between counting the gap (exclusive) and counting both endpoints (inclusive).
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Last reviewed: · Methodology based on US building code standards, contractor pricing surveys, and manufacturer specifications.