I've seen it one too many times in the past week:
I follow a link. There's an entry in some kind of blog, or a column or article in the online version of some kind of newspaper.
AND THERE'S NO DATE ANYWHERE ON THE PAGE.
It might've been published today. It might've been published ten years ago.
There is no way to find out, short of contacting the author and asking them.
And it's got to stop.
If your blogging system doesn't automatically time- and date-stamp your entries, then please for the love of all that's good turn on the configuration option that puts dates on entries. Please.
WON'T SOMEONE THINK OF THE CHILDREN?
Not quite as bad, but has still been annoying me for a long time: blogs that post several long entries per day, but only show the date once; they put the time on each entry, but not the date.
For example, say you want to see whether BoingBoing has covered something that happened two days ago. So you go to their main page, and you try to find the first entry that was posted on the day in question. It's a tedious and error-prone process. (Yes, you can search in the page for the preceding date and then scroll up, but there are a variety of reasons why that's also not a great user experience, including the fact that the dates are displayed in small gray print, making them hard for me to see even when the browser highlights them.) I mention BoingBoing because they're very high-profile, but I actually see this kind of thing quite often in individuals' blogs.
Just put a date-and-time stamp on every entry. It'll make things easier for some readers, and it won't hurt the others.
(Wrote this almost a week ago, but for some reason didn't post it.)