Factorize some functions

Oops, indeed. It does not relate to my first stupid comment, but maybe you should keep the format of the previous code for better uniformity:
The spaces in the concatenation:
<pre>'+' . $nth . ' ' . self::$days[$dayOfWeek]</pre>
And a simple space in the assignation:
<pre>$dt = $this->copy()->firstOfMonth();</pre>
As you did it in the others methods.