The description per Encoder Pro of 99050 states "Services provided in the office at times other than regularly scheduled office hours, or days when the office is normally closed (eg, holidays, Saturday or Sunday), in addition to basic service".
I would think that posting your hours would be a requirement to use this code. As in the situation that the office hours posted are Mon-Fri 8AM-5PM, but patient can not schedule an appointment, has emergency, etc and needs to be seen outside of your regularly scheduled (posted) office hours.
Just be careful not to use this code when a patient is seen outside of normal business hours due to reasons such as the physician is late, physician schedule did not allow prior appointment, or any other reason that could be considered "the physicians/office fault".
Also, keep in mind that most insurance carriers considered this a "non-covered service", which means that you would have to forward to charge for this on to the patient. Due to compliance issues, you would not be able to "write-off or adjust off" this amount when it is patient responsibility due to non-coverage by the insurance carrier.
I don't know if this answered your question, but I hope it helps!!