There is a Java Action in the CommunityCommons available for this conversion called ‘LongToDatetime’
Use the community commons LongToDateTime function.
You do not need to change a value in your database. Create a dataview in your form where you want to show it. That dataview shows a nonpersistent object with a date time field. Use a microflow for this dataview where you pass the long object to the microflow. Then use the community commons to transform that long field to a data time and store it in the just created object and pass that object back to the form.