If you came here looking for how to do the opposite, like me, here's what I came up with:

Where newTime format is "hh:mm a", e.g. "12:00 AM":

const [timeOnly, period] = newTime.split(' ');
const [hoursStr, minutes] = timeOnly.split(':');
const hoursNum = parseInt(hoursStr);
const hours = period === 'PM' ? ((hoursNum % 12) + 12) % 24 : (hoursNum % 12) % 24;

The only edge case it doesn't handle is if newTime === "0:00 PM". Otherwise it works well.

@syntax-junkie 's answer really helped me figure this out!