Using CiviCRM 4.6.33 on Drupal 7.56. I just noticed, while adding a contribution to this person's record, that for a contribution made on December 31 of last year (2016), the contribution date and the activity date for that contribution do not match. The activity date shows as December 31, 2017, which is 10 days from now.
I was going to report this as a bug, but I have checked donations made by other people on December 31, 2016, and they do not seem to have the same problem. I must have done something to make this happen, I suppose, but I see no way to manually change the activity date.
Here are screenshots of the contributions and activities windows.