Data retention date

Counting by calendar rather than by days is the part that matters for a policy. "Seven years" means the same date seven years later, not 2,555 days later — those differ by one or two days depending on how many leap years fall in between, and a deletion job written in days will drift.

Month arithmetic has its own edge, and this clamps it the standard way: one month after 31 January is 28 February, because there is no 31 February. That is the convention almost every date library uses, and it is worth knowing it is not reversible — adding a month and subtracting one does not always return you to where you started.

A point about where the clock starts, which is the thing most often got wrong in practice. Retention rules usually run from the end of an event rather than its beginning: UK company records run from the end of the financial year, not the transaction date; employment records typically run from the end of employment. Getting the start date right matters far more than the arithmetic. And this is calendar maths, not legal advice — the period your policy requires is a question for the policy, and for a lawyer.