As an example:
When did merged Namecoin-Bitcoin mining start?
The event has already occured thus the question seems out of place. One option would be to simply close the question however there is a good chance we will eventually get a past tense version of same question.
I was planning to change the question to reflect that fact that the event is in the past however I wasn't sure on the protocol for this type of change so I thought I would check with the community first.