One Set of Fields or Another Required

[eluser]Aken[/eluser]
That's one way to go about it. Alex is right; there's really no standardized way to do this.

Other options:

1) Create rules based on POST values.

Code:

if ( ! $this->input->post('hours'))
{
    // Require your start/stop times.
}
else
{
    // Require hours
}

2) Don't use form validation at all, and check the values / create error messages manually.