Guides
Plain-language explanations of the date math behind each calculator on this site, for anyone who wants to understand the method, not just get the answer.
How Business Days Are Calculated (and What "3-5 Business Days" Means)
What counts as a business day, how to count them by hand, and why the same shipping estimate takes longer starting on a Friday than a Monday.
How to Calculate the Days Between Two Dates by Hand
The manual method, the inclusive/exclusive counting trap, and why leap years throw off quick math.
How to Calculate Your Exact Age by Hand
Why subtracting birth year from the current year isn't enough, and how to get a full years/months/days breakdown.
Days and Months Between Dates in Excel or Google Sheets
Simple subtraction, DATEDIF, and NETWORKDAYS, including the undocumented function most people miss.
How to Find the Day of the Week for Any Date by Hand
Zeller's Congruence, explained step by step with a worked example.
Why Do We Have Leap Years?
The real astronomy behind February 29th, the century rule, and what would happen if we skipped it.
What Is a Julian Date?
The astronomical definition, Modified Julian Date, and the informal manufacturing usage, untangled.
How Military Retirement and Terminal Leave Dates Work
The 20-year active-duty mark, terminal leave math, and why Reserve and Guard retirement works differently.