For those using StaggerNation’s DateTags plugin with Movable Type 4 (or, presumably, 5): there was a change in the MT3 —> MT4 transition with how entries’ date information is handled. This change can make DateTags behave in odd ways in cases where posts are assigned publishing dates other than the dates on which they were originally written — for instance, if you’re writing entries today and assigning publishing dates three weeks from now, because you’re using DateTags to create a calendar of upcoming events. To address this problem, do a global find-and-replace in your plugins/DateTags.pl file: where you see created_on, replace it with authored_on.

That is all.