Make the result useful
A calculation is easier to trust when you know what it counts. These notes explain the difference between calendar units, elapsed days and inclusive spans, with examples you can try in the tools.
Calendar guides
A little planning for a meaningful day.
Turn a birthday countdown into a simple, manageable plan.
Calendar guidesMonths do not all have the same shape.
How month lengths affect anniversaries and date arithmetic.
Calendar guidesWrite dates so they are easy to read.
A few small choices help prevent month-and-day mix-ups.
Calendar guidesA date is not a timestamp.
Why the tools count calendar dates without asking for a time zone.
Calendar guidesA year is more than 365 days.
Understand why calendar age and total days are different ways of describing the same span.
Calendar guidesAre you counting the gap or the dates?
One simple distinction explains many one-day differences between results.
Calendar guidesThe extra day, explained.
A short guide to February 29 and the Gregorian leap-year rule.
Calendar guidesGive a milestone a little context.
A useful personal timeline is more than a list of dates.