The rules behind
the dates.

How courts, banks, carriers, and contracts actually count days — explained properly, with calculators that follow the same conventions.

Deadlines are missed on conventions, not arithmetic. Does the clock start today or tomorrow? Do weekends count? What happens when the last day is a Saturday, or when a month is short? Different institutions answer differently, and the difference is often the whole dispute. DateSum documents those conventions and computes them exactly.

Guides

How to Count Business Days (Without Missing a Deadline)

The rules banks, courts, and couriers use to count business days — including what happens with weekends, holidays, and cut-off times.

The 90/180-Day Schengen Rule, Explained With Examples

How the Schengen short-stay rule actually counts days, why the 180-day window is "rolling", and how to compute your remaining days.

Why Easter Moves: How the Date Is Actually Determined

Easter can fall anywhere from March 22 to April 25. Here's the real rule, the ecclesiastical moon, and how to compute it.

90-Day Probation Periods: How to Count Them Correctly

Whether a probation period is 90 calendar days or 3 months matters more than you think. How employers and employees should count.

Leap Years, DST, and Other Ways Date Math Goes Wrong

The five classic bugs in day counting — leap years, DST, month lengths, inclusive counting, and timezones — and how to avoid each.

Net 30, Net 60, Net 90: How Invoice Payment Terms Really Work

What net 30, net 60, and net 90 really mean: when the clock starts, the 2/10 discount math, late-fee conventions, and how to set terms that get you paid.

Return Windows: How 14, 30, and 90-Day Policies Actually Count

When does a 14, 30, or 90-day return window actually end? How retailers start the clock, why calendar days rule, and how to count your deadline safely.

Pregnancy Weeks to Months: How the 40-Week Calendar Works

Why pregnancy is counted as 40 weeks from the last period, how weeks map to months and trimesters, and how a due date is calculated with Naegele's rule.

All guides →