12:00 AM is the internal default time set by a geodatabase when there is no user input for time in a date field. If 12:00 AM is manually input into a date field, ArcGIS handles it as if it were not entered since it matches the default value and is not displayed in the date field. This is by design.
A geodatabase formats the date as 'datetime yyyy-mm-dd hh:mm:ss AM or PM'. By default, when a user does not input a time, the date field value is set to 12:00 AM. Similarly, when a user does not input a date, the date field value is set to December 30, 1899.
For more information, please refer to the Dates and time section in the ArcGIS Resource Center: SQL reference for query expressions used in ArcGIS