Ah ok - that would show up in the User object, for example if you call users/show.json:
$ twurl "/1.1/users/show.json?screen_name=andypiper"
{
"id": 786491,
"id_str": "786491",
"name": "andy piper (pipes) 🤓🕸🏳️🌈⌨️",
"screen_name": "andypiper",
[...]
"created_at": "Wed Feb 21 15:14:48 +0000 2007",