d.o.b. is a written short form of date of birth — the day, month, and year you were born. You almost only see it on forms: school, hospital, passport, job papers. You write your birthday in the past, not the day you fill in the form. People almost never say “d.o.b.” out loud; they say “date of birth” or “when were you born?” The dots show it is an abbreviation, not a word. Different countries write the date in different orders (day-month-year, or month-day-year), so the form often tells you which order it wants.

出生日期（表格欄位；台灣證件常寫「出生年月日」）