 Re: What is correct
Right.
To me there is no conflict. Zoe is notifying her colleague/client that an action had been taken on a certain date. As this may be part of an ongoing series of communication, it could be "had replied ... on/by xxx" or "replied ... on/by xxx". We do not have the full context, so it is a personal choice which to go with. Maybe Zoe would like to say what she has used or would prefer to use.
As to the preposition, it could be argued that it is not clear if the action was by a date or on a date, and in my view that is not affected by tense. |