Dates, Ages, and Durations: The Math People Get Wrong
Inclusive counting, month-length traps, time zones, and why your age changes at midnight — the conventions behind date calculators.
Counting days: in or out?
"From Monday to Friday" is 4 or 5 days depending on the convention. Date math counts the difference by default (4), and adds one for inclusive counting (a hotel stay). The date calculator exposes both modes — decide which question you are asking before reading the number.
Months are not a unit
"In three months" lands on different day counts depending on the start. Serious date tools compute months first, days second — the same years-then-months-then-days convention used by the age calculator on official documents.
Work hours are not clock hours
A work hours calculator skips breaks, weekends, and holidays — a "40-hour week" is policy, not arithmetic. For cross-timezone meetings, convert both sides to UTC first; the time zone calculator removes the mental math, and daylight saving means the same offset is not stable all year.
Durations worth memorizing
A million seconds is 11.6 days; a billion seconds is 31.7 years. The day counter and duration calculator make these conversions routine for deadlines, warranties, and visa stays.