Yes, but you have to round to the nearest 15 minutes - I would guess you want to round down (floor calculation)
DATETIME(INT(FLOAT([Date]) * 24 * (60/[Bucket Size])) / (24 * (60/[Bucket Size])))
In this case [Bucket Time] is a parameter in minutes that could just be set to 15.
Is there a resource at all for floor and ceiling calcs in Tableau? I don't do them often enough to have a reference handy, so every time I do I have to make one up or hopefully find something.