Why 'Add 30 Days' Never Lands Where You Expect

It's an easy mental shortcut: '30 days from now' feels like it should mean 'the same date next month'. But months aren't 30 days long — they range from 28 to 31 — so the two answers only coincide by coincidence, and usually don't.

This matters more than it sounds for anything with a real deadline attached. A '30-day notice period' starting on January 20th ends on February 19th, not February 20th — a full day earlier than the 'next month, same date' intuition would suggest, and the gap gets larger or smaller depending on which months are involved.

The safest habit for anything with legal, contractual or financial consequences (notice periods, payment terms, warranty windows) is to count the actual number of days explicitly rather than eyeballing a calendar month-to-month — which is exactly the gap a tool like this is built to close instantly.

The same logic applies to subtracting days: 'the date 90 days before delivery' isn't 3 calendar months earlier by default, since months vary in length — the exact answer depends on precisely which 90-day window is being counted through.