WhenMathCalculate when. Understand what changes it.
WhenMath answers ordinary questions about dates and time, the kind that look simple until a weekend, a holiday, a month-end, or a time zone quietly changes the answer. Every result comes with the assumption that produced it, so you know exactly what you are looking at, not just a number.
Why "three weeks from now" is not always the same date twice
Most date and time arithmetic looks trivial until it isn't. Adding a month to January 31 does not have one obvious answer. Counting "10 business days" depends on whether the start date counts, which holidays apply, and which days of the week your team actually works. A 9 AM meeting converted across three time zones can land on a different calendar date entirely, or inside an hour that does not exist because of a daylight-saving transition. A generic calculator gives you a number and hides the convention it used to get there. WhenMath treats the convention itself as part of the answer.
How a WhenMath result works
Answer
The direct result, above the fold, with sensible defaults.
Understand
A calculation audit: which days counted, which were excluded, and why.
Stress-test
See what changes the outcome, worked out deterministically, not a generic guess.
Plan
Dependencies and buffers become a usable timeline, not just one figure.
Act
Copy, share, or export the result to where it actually needs to live.
Nine ways time gets complicated
Every WhenMath engine belongs to one of nine categories, grouped by the kind of timing decision they solve rather than by keyword. Browse a category to see the conventions and edge cases that matter within it.
Core Time Math
Durations, totals, conversions, and overlaps within a clock, not a calendar.
Date & Calendar Math
Differences, offsets, and structure across the Gregorian calendar.
Business Days & Work Calendar
Counting and placing dates against a working-day calendar, not a plain one.
Deadlines & Project Capacity
Turning a deadline, an effort estimate, or a phase list into a working plan.
Work Hours, Timecards & Shifts
Turning clock-in and clock-out times into accurate paid and elapsed hours.
Recurrence & Events
Generating and reasoning about dates that repeat on a rule.
Time Zones & Meetings
Coordinating a single instant, or a shared window, across IANA time zones.
Age, Anniversaries & Life Dates
Ages, anniversaries, and other durations measured against a life date.
Active Time Tools & Daylight
Running timers, and the sun’s position, for a specific place and day.
Nothing you type leaves your browser
WhenMath runs entirely in your browser. There is no account, no server-side calculation, and no history saved anywhere but your own device. Read more on the Methodology and Privacy pages.