Date Calculator
Find the number of days between two dates, or add and subtract days from any date.
What is a Date Calculator?
A date calculator is a tool that helps you work with calendar dates in two distinct ways. The first mode finds the exact number of days, weeks, months, and years that separate two dates — useful for counting down to an event, figuring out how long a project took, or determining someone's age in days. The second mode adds or subtracts a number of days from a starting date and tells you the resulting date — useful for scheduling deadlines, computing expiration dates, or finding what day of the week a future date falls on.
Unlike a simple subtraction, a good date calculator also gives you business-day counts (excluding Saturdays and Sundays), breaks the total into weeks and remainder days, and provides approximate month and year equivalents — all of which are hard to compute in your head but instantly useful once you have them.
How Days Between Dates Are Calculated
The number of days between two calendar dates is simply the difference between
their Julian day numbers — or equivalently, the number of 24-hour periods that fit
between midnight of the start date and midnight of the end date. In most programming
environments this is computed by subtracting one date from the other to get a
TimeSpan
and reading its TotalDays
property. Leap years, months of varying length, and daylight saving time transitions
are all handled automatically because the underlying arithmetic operates on absolute
timestamps rather than calendar notation.
What Are Business Days?
Business days (also called working days) are weekdays only — Monday through Friday. Saturdays and Sundays are excluded. Public holidays are not excluded by this calculator because they vary by country, region, and employer. The business-day count is therefore a maximum; your actual working days may be somewhat lower if the span includes holidays.
The fastest way to count business days is the "full weeks" formula: divide the total days by 7 to get full weeks (each contributing 5 business days), then count the weekday contribution of the remaining partial week by checking each day individually. This runs in constant time rather than iterating over every single date in the range.
Months and Years: Why Approximations?
Calendar months have different lengths (28, 29, 30, or 31 days), and years have either 365 or 366 days. This means there is no single conversion factor from days to months or years. The calculator uses 30 days per month and 365 days per year as approximate values — the same convention used in most financial and legal contexts. For an exact month-and-year count (e.g., "2 years, 3 months, 14 days") you would need to walk the calendar forward from the start date, which is a different calculation that depends on which specific months are involved.
Example — January 1 to December 31
From January 1 to December 31 of the same non-leap year there are 364 calendar days (52 weeks exactly). Of those, 260 are business days and 104 are weekend days. In approximate terms that is about 12 months or just under 1 year.
If you add 90 days to January 1 you land on April 1 (in a non-leap year) — a quick way to estimate: 90 days ÷ 30 ≈ 3 months, so roughly the start of April. The exact result depends on whether February in the range is 28 or 29 days.
About These Parameters
- Start Date / End Date (Difference Mode)
- The two calendar dates you want to compare. The order does not matter — the calculator always returns a positive difference. You can enter past or future dates, or even two dates in different years. The result is computed in UTC midnight timestamps so daylight saving time transitions do not affect the count.
- Start Date (Addition Mode)
- The date you want to start from. The calculator adds or subtracts the specified number of calendar days and returns the resulting date together with the day of the week it falls on.
- Number of Days
- The number of calendar days to add or subtract. Enter any positive integer. The direction toggle (Add / Subtract) determines whether the days move forward or backward in time.
- Direction
- Choose Add to move forward in time (find a future date) or Subtract to move backward (find a past date).
Frequently Asked Questions
Does the calculator include or exclude the end date?
The calculator counts the number of whole days between the two dates, which means it excludes the end date itself. This is sometimes called "exclusive end" counting. For example, from January 1 to January 5 there are 4 days (Jan 1, 2, 3, and 4 — January 5 is not counted). This matches how most everyday date differences work: a trip from Monday to Friday lasts 4 days, not 5, because you arrive on Friday rather than staying an extra night.
How do I calculate how many days until a future event?
Use the Date Difference mode and enter today's date as the Start Date and the event date as the End Date. The calculator will show the total number of days remaining. If you want to know what date something falls on after a waiting period, switch to Add / Subtract Days, enter today as the Start Date, and add the number of waiting days.
What is the difference between calendar days and business days?
Calendar days count every day including weekends and holidays. Business days count only Monday through Friday, excluding weekends. They do not automatically exclude public holidays, which vary by country and employer. When a contract says "within 30 business days," that is equivalent to roughly 6 calendar weeks, not 30 calendar days.
Why might my result differ from another date calculator?
Different calculators use different conventions for whether the start date, end date, or both are included in the count. This calculator uses "exclusive end" counting (end date not included), which is the most common convention in software. If another calculator uses "inclusive" counting it will show a result that is 1 day higher. For the month and year approximations, different tools may use 30.44 days per month (the calendar average) or exactly 30 days — resulting in slight differences for large date ranges.