The month-end rule is why this set exists
Adding a month is not adding 30 days, and the difference shows up at the end of the month. One month after 31 January is 28 February here — 29 in a leap year — not 3 March. We clamp to the last valid day of the target month rather than letting the overflow roll forward, because that is what every legal and accounting definition of "one month" means and it is what almost everyone intends.
This is a place where our answer legitimately differs from some other calculators', so every page in this set states the rule and shows whether it actually bit on that page. One consequence is worth knowing: the operation is not reversible. 31 January plus one month is 28 February, but 28 February minus one month is 28 January.
What each page gives you
The exact date and weekday, the day count the span actually contains — which varies between 28 and 31 days per month, so the total is never a round multiple — the business-day equivalent, the ISO week, and any holiday, leap day or clock change inside the span. Each page links to its mirror in the other direction and to the equivalent day count, which is usually a day or two different.