Date validation CI 1.7

[eluser]codex[/eluser]
This has me puzzled for days so I hope someone can shed some light on the issue.

Suppose you'd like to validate a user's date of birth, which consists of 3 dropdowns 'day, month, year'. All 3 combined make up 'date_of_birth'. If one of the 3 selects is not filled in, the rules that apply to date_of_birth spring into action. (does this make any sense?)

You can't do

Code:

$this->form_validation->set_rules('date_of_birth', 'Birthday', 'required');

since date_of_birth isn't an input and is therefore not posted.

How do you check something like this?