Establishing a publication date has two aspects. It must be assessed separately whether a given date is indicated correctly and whether the content in question was indeed made available to the public as of that date.
The nature of the internet can make it difficult to establish the actual date on which information was made available to the public: for instance, not all web pages mention when they were published. Also, websites are easily updated, yet most do not provide any archive of previously displayed material, nor do they display records which enable members of the public - including examiners - to establish precisely what was published and when.
Neither restricting access to a limited circle of people (e.g. by password protection) nor requiring payment for access (analogous to purchasing a book or subscribing to a journal) prevent a web page from forming part of the state of the art. It is sufficient if the web page is in principle available without any bar of confidentiality.
Finally, it is theoretically possible to manipulate the date and content of an internet disclosure (as it is with traditional documents). However, in view of the sheer size and redundancy of the content available on the internet, it is considered very unlikely that an internet disclosure discovered by an examiner has been manipulated. Consequently, unless there are specific indications to the contrary, the date can be accepted as being correct.