ez24: I'm glad you found some solutions to your problem.
I guess it depends at the end of the day what you're trying to prove. When it comes to proving date and time, usually a time stamp (such as on CCTV) is good enough. Even if the time/date is completely wrong (or not even set), it doesn't necessarily render the evidence inadmissible (even in murder trials), it all just depends on what other evidence you have. If you're being accused of something, it's not simply good enough for the prosecution to say "oh, your time stamps were wrong", they need to provide evidence to that effect. You, as the defendant in a matter could simply say, "well no, the time stamps are correct. I set the clock myself (or it's set Automatically via NTP) and look, here is the CCTV box, it still shows the correct time/date just as it did when the video was recorded".
A sundial could be a good way to provide "redundancy" to a burnt-in date/time or metadata to be able to reliably and fairly accurately show the time of day and rough day of the year (through sunrise/sunset times). You don't even need to go out and spend money, everyday objects will do like trees, letter boxes etc... anything that casts a shadow.