was a day

If someone says that a person is [some age] if they are a day it is an idiomatic way of expressing the opinion, about a person of unknown age, that they are at least the age mentioned, and probably older. A decidedly old fashioned expression, probably more common in British English. Often said to someone who supposed the person to be younger.

Mrs Jones is fifty if she's a day - If she is over one day old (i.e. any age at all) then she is fifty.

Stella: Jane Taylor tells me that she is going to retire at the end of the year.
Ralph: Really! She must be taking early retirement, she’s only about fifty.
Stella: No way! She’s sixty if she’s a day! She went to the same school as my Dad and he retires next year.

The expression is common in American usage, too.

Excellent answers. Thank you.