Every calculator on this site documents the rules it follows, but some of those rules need more room than a tool page can give them. These guides are the long version: what the standard actually says, where the edge cases are, and why the conventions ended up the way they did.
They are written for people who need to get a date right, not for people studying horology. Where a rule has a genuine ambiguity — how to count a month from 31 January, when a leap-day birthday falls, whether a week number starts on Sunday or Monday — the guide says so and explains which convention the tools here use, so you can check a result instead of trusting it.