I try to give date information whenever I can. Digital photographs have the date contained in the EXIF data and this is read by the Zenfolio software and displayed following the title "Taken" in the top left hand corner of the image page. However, in the case of scanned images there is no EXIF data and it displays the date of the file, which is not the date of the photograph. I have therefore adopted the following convention so that you know which date to believe!
Digital photos always use the EXIF data and display it with the title "Taken". I don't then normally put a date in the caption.
Scanned images will have no EXIF data. On the right hand side they will say "No EXIF information is available" which is how you determine that they are scanned images. If I have date information I will put it in the caption, except where the image is one clearly taken at the same time as the previous photograph.
When a gallery is composed entirely of scanned images I will sometimes disable the "Date taken" display to avoid confusion. At the moment this has mainly been done with the Bradford trolleybus gallery.
If you look at the EXIF data that will also give the time of the photograph. I always now set my camera on GMT and leave it at that as that remains constant and adjustments can always be carried out to correct to local time afterwards. In practice I couldn't guarantee changing the time zone at exactly the right time in autumn and spring and also whenever i travelled to or from abroad. This does not apply to photographs from 2006 and a few in July 2010 (new camera syndrome!).