<p>Sounds easy, but I can't find where this built in class and others exists in the documentation. I use functions but want to know what there is on the OO side.</p>

It's not a URL or anything, but you can get a fair idea using Reflection:
```
```
Reflection::export(new ReflectionClass('DateTime'));

Class [  class DateTime ] {

  - Constants [11] {
    Constant [ string ATOM ] { Y-m-d\TH:i:sP }
    Constant [ string COOKIE ] { l, d-M-y H:i:s T }
    Constant [ string ISO8601 ] { Y-m-d\TH:i:sO }
    Constant [ string RFC822 ] { D, d M y H:i:s O }
    Constant [ string RFC850 ] { l, d-M-y H:i:s T }
    Constant [ string RFC1036 ] { D, d M y H:i:s O }
    Constant [ string RFC1123 ] { D, d M Y H:i:s O }
    Constant [ string RFC2822 ] { D, d M Y H:i:s O }
    Constant [ string RFC3339 ] { Y-m-d\TH:i:sP }
    Constant [ string RSS ] { D, d M Y H:i:s O }
    Constant [ string W3C ] { Y-m-d\TH:i:sP }
  }

  - Static properties [0] {
  }

  - Static methods [0] {
  }

  - Properties [0] {
  }

  - Methods [9] {
    Method [  public method __construct ] {
    }

    Method [  public method format ] {
    }

    Method [  public method modify ] {
    }

    Method [  public method getTimezone ] {
    }

    Method [  public method setTimezone ] {
    }

    Method [  public method getOffset ] {
    }

    Method [  public method setTime ] {
    }

    Method [  public method setDate ] {
    }

    Method [  public method setISODate ] {
    }
  }
}
```
```
Not perfect, as there are no arguments, but a starting point at least.  They should make this easier to find out, I agree!