After casting Hand of Guldan, it starts correctly showing number of imps, but they expire much faster (i guess because of haste and 5 firebolt casts limitation) and after they expire, it is 2-3 seconds, when icon incorrectly shows that there are imps still, but they already expired.

Posted by CurseForge user tonibexi | Imported from CurseForge issue #1649 | Raw

Yeah, TMW is only trying a best guess at their duration. There are factors that go into their duration that addons simply don't have access to, so I can only do the best with the information that is available. See https://wow.curseforge.com/projects/tellmewhen/issues/1539

Posted by CurseForge user Cybeloras