Which calculator for which situation
- Counting time between dates. Days between dates for the total interval; business days when a deadline excludes weekends; age and age difference in exact years, months and days.
- Projecting a date. Add or subtract dates to find a due date from a term; countdown for a scheduled event.
- Hours. Add hours for timesheets and time tracking.
- Calendar. Day of the week for any date, past or future; leap year to check the full rule.
Why counting days is harder than it looks
Dates are the classic source of silent errors in spreadsheets and software, for four reasons that stack:
Months are different lengths. "One month from" 31 January has no obvious answer — it could be 28 February, 2 March or 3 March depending on the convention. Contract terms need to say whether they count in days or months.
The leap year rule has three parts. Divisible by 4, except by 100, unless by 400. Implementing only the first works perfectly between 1901 and 2099 — and fails at 1900 and 2100.
Inclusive or exclusive endpoints. From 1 to 10 March, is that 9 days or 10? It depends on counting the start day. In legal deadlines and leases, that choice moves the due date.
Time zones. A date with no time is read as midnight, and midnight in UTC can be the previous day locally. It is the error that makes a birthday appear a day early.
Calendar days and business days
The distinction carries consequences. Court and administrative deadlines frequently run in business days, while private contracts usually run in calendar days unless they say otherwise. Public holidays vary by state and even by city, which is why a deadline computed remotely can differ from the local answer.
Frequently asked questions
How do I calculate exact age? Subtracting years is not enough: you have to check whether the birthday has already occurred this year. Someone born in December is, in January, a year younger than subtraction suggests.
How many days are in a "commercial" month? Thirty, by convention, in financial and payroll calculations. It is not the calendar month — it is a simplification that standardises interest and proportions.
How do I add hours that cross midnight? Treat everything as total minutes and convert at the end. Adding 22:30 and 3:45 gives 26:15, which is 2:15 the next day — the add-hours calculator keeps totals above 24 when that is what you want.
Does the calculation account for daylight saving? Date arithmetic works in whole days, so DST does not affect it. It matters only for sums of hours that cross the changeover.