Trying to create rules/event based on item DateTime

[rossko57]

Recent relevant post -

[SOLVED] Rule should fire at the date of a DateTime item

I have a DateTime-Item, which gets populated externally: .items DateTime Event_StartTime now I want to perfom an action at the exact time given in that item. and only once… Is there a simple way in a rule for this? Or is it only possible with a cron based rule and the help of a proxy-item, which sets “event fired” on TRUE, if the event was already fired?