I can’t remember if I ever mentioned that almost all the birthdays in One Piece are number puns, but they are. Basically: most number kanji in Japanese have approximately a bazillion ways they can be read depending on the situation, so if you’re bound and determined, you can wrangle a number out of almost any word or name by finding sounds in it that match with a certain number.
So here’s the birthdays of the Water 7/CP9 folks! They’re all number puns except for Kalifa’s (hers is Administrative Professionals Day).
Iceburgs comes from アイス/aisu, the first half of his name. Since アイ/ai sounds like the English letter ‘I’ and ‘I’ looks like the number 1, he’s January. Meanwhile we get ‘3′ from ス/su (I'm still not fully sure why on that particular one, actually?) for 1/3.
Franky’s birthday comes from the first and last sounds of the word サイボーグ/saibougu (cyborg). サ/sa is one of the most common readings of ‘three,’ while グ/gu can be interpreted as a rendaku’d ク/ku, which is ‘nine.’ So 3/9
Paulie’s birthday is from the word 縄/nawa (rope). な/na is from one of the most common readings of 'seven', and 'eight' is frequently read as は/ha which is in turn frequently pronounced as wa- so 7/8.
Lucci's just comes from ロブ/robu, the katakana version of his name 'Rob.' ロ/ro is from the most common reading of 'six' and ブ/bu again can be interpreted as a rendaku, in this case of フ/fu which is a reading for 'two.' 6/2.
Kaku's is my favorite here. His birthday is 8/7, because as mentioned, 8 is often read as は/ha and 7 is often read as な/na. 鼻/hana means 'nose.'