Column names - (created, modified) vs. (creation, modification)
Of those two options creation/modification as that is less ambiguous.

Looking at some SQL or documentation for the structure that doesn't include column type information, created/modified could be mistaken for boolean values.

Furthermore, I'd be inclined to be more specific and use creationTime/modificationTime or createdTime/modifiedTime, probably the latter because it is vaguely quicker to type.