Thank-you Sent same as Recieved Date
There is an open issue related to your question https://issues.civicrm.org/jira/browse/CRM-20894. It also happens to me with a recent upgraded CiviCRM from 4.8 to 4.7.22 and it can also be replicated in dmaster.

[EDIT]

Seems it's already solved here https://issues.civicrm.org/jira/browse/CRM-20773