The age calculator works out how old someone is from a date of birth, giving the answer in completed years, months and days rather than as a decimal. It counts the way people and institutions actually count: a birthday adds a year on the day itself, so someone born on 3 June turns a year older on 3 June and not the day before or after.
The awkward case is 29 February. A person born on a leap day has no anniversary in three years out of four, and different systems resolve it differently — some roll the birthday to 28 February, others to 1 March. In England, Wales and Northern Ireland the common law convention for reaching a legal age treats the anniversary as falling on 1 March in non-leap years; Scots law has generally taken 28 February. This calculator reports the completed interval and shows the assumption it has used, so the figure can be checked rather than taken on trust.
Months are handled by calendar rather than by an average length. From 31 January to 28 February is one month, not 28 thirtieths of one, and the remainder in days is measured from the last complete month boundary. That is why an age in years, months and days converted to a plain day total will not always match a straight subtraction of the two dates — both figures are correct, they are answering slightly different questions.
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