If he’s, she’s, etc. a day
Definition 1 of 1
Pro Tip 1/3Emphasizing Certainty
This phrase strengthens the speaker's certainty about someone's age, often used when guessing or estimating. 
She must be 30 if she's a day, given her career accomplishments.
Pro Tip 2/3Age Exactness
The speaker implies that the person is at least the mentioned age, if not older. 
He's 70 if he's a day, judging by the stories he tells.
Pro Tip 3/3Subtle Surprise
It's often used to express a subtle surprise about someone's age, which appears younger or older than expected. 
He's been teaching here for 40 years if he's a day—can you believe it?