Guides
The rules behind the numbers — how institutions actually count days, and where counting goes wrong.
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.
Billing Cycles: Why 'Monthly' Isn't Always 30 Days
Why monthly billing runs 28-31 days: calendar-month renewals, date clamping, credit card grace periods, proration, and honest annual-vs-monthly price math.