1 Month Old?

Okay, this is going to be a pretty silly/dumb question but for some reason I am getting so confused... my son was born on 3/27. Today he is 4 weeks old. Does that make him one month old today? Or will he be one month old on the 27th? I thought he'd be a month on the 27th but my husband said he's one month old today. Please don't make fun, I just really can't figure it out! 😔😣