if a day

if a day AND if it / one is a day (a phrase attached to an expression of someone's age, often expressing a belief that someone is older than they want to be considered) — ни больше ни меньше, ни много ни мало, как раз, как никак (преим. о возрасте)

Example 1: He can't be thirty. I'm sure he's forty-five if he's a day. — Да ему все сорок пять, ни больше ни меньше.

Example 2: You're thirty if you're a day. — Вам никак не меньше тридцати.

Example 3: She is ten years older, than you if she's a day.— Да она на добрых десять лет старше тебя.

Example 4: That was a good thirty years ago if it was a day. (J. Joyce, ‘Dubliners’, ‘The Dead’) — Это было лет тридцать тому назад, ни больше ни меньше.

Note
The speaker is expressing what he believes to be a highly accurate estimate of the subject's age. It may or may not be in response to the subject claiming to be younger or "not acting his age." (usingenglish.com)