If all you want to do is clarify, "I meant to confess to committing this crime on date #1, not date #2 when I was not present, and I want to be sure that we're all on the same page - this diversion is for the date I was present," the prosecutor may well note that in the file. If this is pretrial diversion, you didn't plead guilty in court, so as long as the prosecutor doesn't intend to charge you for both dates it doesn't much matter if you got the date wrong. If you violate the terms of the diversion program you can be sure that when you go back to criminal court they use the correct date.

If you are hoping that the pretrial diversion case will go away if you point out your mistake to the prosecutor, that's not going to happen. If you come across as having a bad attitude, you could conceivably inspire the prosecutor to agree that you should not be in a diversion program for date #2, release you from the program, and charge you with your admitted crime on date #1 without again agreeing to diversion.