If you use the word "or", instead of "and" here, your problem will automatically be solved.
There are two scenarios when the reader need to request for a date/time change:
- The reader may find the date does work him/her, then you need to change both the date and the time for him/her. (In this case, date and time can be viewed as one item, because they are bound together. Once the date changes, the time automatically changes.)
- The reader may find the date is acceptable, but the time does not work for him/her.Then, you can keep the date and change only the time. (In this case, date and time are viewed as two items.)
I suggest you edit the sentence to "If this date or time is not convenient please contact me at..." So, the reader has the freedom to change either the date, or the time. The time will of course be rescheduled once the date is changed. Then, it would be obvious here for you to use "is" here.