Email Template and Date Time Merge Fields shows date without time
|Knowledge Article Number||000006726|
There are Date fields in Salesforce such as Message Date that will display in the UI with the Date and Timestamp but when used in an Email Template for example only show the Date. That is because the field is not a Date/Time field, but a Date field.
Date and date/time fields cannot be used interchangeably. The name alone may not indicate if a field is a date or date/time. For example, Created Date and Last Modified Date are date/time fields whereas Last Activity Date is a date field.
This explains why the time may not show up when merging a field that shows the date AND time via the UI.
For managing the opposite issue, needing a Date/Time field to only display the Date in a merge, use the DATEVALUE function to convert a date/time field into a date field.