if (one) is a day
Used to emphasize that one must be a certain age. How does he keep himself looking so young—he's got to be 60 if he's a day!
if he (or she) is a day
at least (added to a statement about the age of a person or thing). 1992 Shashi Tharoor Show Business Lawrence must be fifty if he's a day.
ˌif he’s, she’s, etc. a ˈday
(informal) (used when talking about somebody’s age) at least: She isn’t forty! She’s fifty-five if she’s a day!