For bars up to and including daily, it’s bar start time.
?
But I think (though I’d have to go and check, which I can’t at the moment) that for weekly and monthly bars it’s the end date that’s returned. And, if I remember correctly, for the current week/month it’s today’s date.
?
You know, you really should check this out for yourself, or at least verify what I said. I may well be remembering incorrectly.