Date Calculator
Use one transparent engine to measure a date span, move a date forward or backward, count business days, or compare local date-times across time zones.
Standard elapsed-date counting excludes the start boundary. The result is shown as an absolute span.
Context matters.
- A day-only calculation uses Gregorian calendar dates, not 24-hour timestamps, so daylight-saving transitions do not change this result.
- Use the Business Days mode when weekends or U.S. federal holidays must be excluded.
Sources & scope
The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.
Six common date calculations.
Open a worked example with safe sample values already filled in. Every field remains editable, so the example becomes your calculation in one step.
How many days are between two dates?
Measure a 90-day example, then replace either date and choose elapsed or inclusive counting.
Calculate the difference 02 Calendar dateWhat date is 90 days from today?
Move forward by 90 calendar-day boundaries with the starting-date convention kept visible.
Add 90 calendar days 03 Working deadlineWhat date is 90 business days from today?
Exclude Saturday, Sunday and observed U.S. federal holidays, or change the calendar yourself.
Add 90 business days 04 Month-end ruleWhat is January 31 plus one month?
See why a missing day in February needs an explicit clamp or end-of-month rule.
Try the month-end example 05 Time durationHow long is 9:45 AM to 5:30 PM?
Calculate hours and minutes between two clock times, with an optional break deduction.
Calculate the hours 06 Time zone & DSTWhy can two clock hours equal one elapsed hour?
Compare New York wall-clock time with true elapsed time across the spring-forward change.
See the DST exampleOne engine, four explicit kinds of date math.
The mode selector changes the inputs and rules, while every result keeps the chosen convention visible. Date-only modes never pass midnight through a time zone; date-time mode resolves each local clock reading to an instant.
- 01Choose the job
Select days between dates, date addition/subtraction, business days, or date-time and time-zone difference.
- 02Set the convention
Choose endpoint inclusion, month-end behavior, weekend days, holidays, time zones and repeated-time handling where they apply.
- 03Check the result trail
Read the primary answer together with calendar units, skipped dates, clock-change effects and the visible source scope.
Date-only calculations use Gregorian year-month-day values. A daylight-saving shift cannot turn a one-day boundary into 23 or 25 hours.
Month and year arithmetic can clamp to the last valid day or preserve end-of-month status when the starting date is already month-end.
All inputs and calculations stay in the browser. No account or server upload is required.
Rule set 2.0.0. Last reviewed 2026-08-22. Next review due 2027-02-22.
What to know before using the result.
What can this date calculator calculate?
It measures days and calendar units between dates, adds or subtracts days, weeks, months, years or business days, counts business days in a range, and compares date-times across IANA time zones.
Does the date calculator include the start date?
Not by default for an elapsed date span. Turn on inclusive counting to count both endpoints. Business-day mode has separate start and end switches so the convention is never implicit.
How does the calculator handle January 31 plus one month?
The default clamp policy returns the last valid day of February. The preserve-end-of-month option keeps a month-end starting date at month end in the target month.
Are my dates uploaded?
No. The calculation runs locally in your browser. Date-time mode uses the time-zone data already supplied by the browser.