
If he’s, she’s, etc. a day
Emphasizing 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.
Age 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.
Subtle 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?
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