Mine was born April 30th, so I just count by the month. She's 5 months old today. It doesn't matter how many days each month has.
You're missing the point. Her LO was born on the 31st and since not all months have a 31st it is not as obvious. This isn't like a week vs months questions. OP, I'd say she is 6 months on the 30th.
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Happy 1/2 birthday! Yay bithday buddies! DS2 was born on March 31 too. I take pictures every month so I am very familiar with this problem. I usually document things on the 1st of the next month as it is what comes after the 30th just like the 31st would. IMO counting the 30th is a day short regardless of being the end of the month. Either way is fine though just whatever you pick you should be consistent since it happens every other month.
Re: If your baby was born on the 31st...