Create a profile with Contribution >> Thankyou Date and use that profile in Find Contributions >> Batch Update via Profile.
Any rows where you do NOT enter a specific value for the Thank-you date have civicrm_contribution.thankyou_date set to "0000-00-00 00:00:00" - rather than NULL. This is a problem because when you view the contribution after the update, the Thank-you Date value is displayed as "0".
I haven't tested this with other contribution date fields (receipt_date, cancel_date, etc.). Please verify whether all date fields have the same problem and fix if needed.