Years forward and years back, and the weekday drift
A date moves forward one weekday per common year and two per leap year. That is why your birthday is on a different day each year, why it sometimes skips one, and why "one year" and "365 days" are not always the same date. Each page in this set computes the drift for its own span and names the leap years inside it — a fact that is genuinely non-obvious, different for every count, and impossible to produce by swapping a number.
Year arithmetic clamps the same way month arithmetic does, and it only ever shows up on one date: 29 February plus one year is 28 February. If your span starts on a leap day, that is the answer you will get here.
Why "one year" and "365 days" drift apart
Add 365 days to 1 March 2027 and you land on 29 February 2028. Add one year and you land on 1 March 2028. Both answers are correct; which one you want depends entirely on the rule you are working to. Anything written in years — a warranty, a retention period, a licence renewal, an anniversary — is calendar arithmetic and clamps at the month end. Anything written in days is a plain count and does not care what the months are called. The two agree whenever the span contains no 29 February and separate the moment it contains one, which over a ten-year offset happens two or three times depending on the date you start from. Every page in this family prints both numbers for that reason, rather than making you guess which convention produced the one number on screen.
What these pages are not
They are arithmetic, not advice. A deadline expressed in years is set by a contract, a statute or a policy, and that document — not this site — decides whether the start day counts, whether the anniversary itself falls inside or outside the window, and what happens when it lands on a weekend. We can tell you which date is ten years from today. We cannot tell you whether that is the date your rule means. The methodology page states every convention used here so you can check ours against yours, and the terms of use says the same thing in fewer words.
What each page gives you
The exact date and weekday, the total day count including leap days, the business-day equivalent, the ISO week, and every 29 February the span contains. Each page links to its mirror in the other direction, to the day-count equivalent and to the leap year calculator. For an exact age rather than a plain offset, use the age calculator, which counts whole years, then months, then days.