Serious question: do people use time_t for representing a date of birth?

To me that wouldn't seem right: a date of birth isn't a timestamp and you typically receive it without a corresponding place or time zone so there's no reasonable way to convert it into a timestamp.

(The other problem is that a signed 32-bit time_t only goes back to 1901. You might not have to deal with a date of birth before 1901 today, unless you're doing genealogy, of course, but until fairly recently it's something you'd probably want to be able to handle.)