Best way to enter time in an optional 12 hr format?

What's the best way for entering time in a 12-hour format?

  1. It's easier to select from a radio button AM / PM but since this field is optional, what if they decided not to fill it out, they cannot unselect the radio button.

  2. Here they're able to unselect AM/PM but defaulting to 'Select' but it's not as fast as just tapping a radio button.

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Which is better? Or any other suggestions?