You could just solve that with a microflow. Use the on change on a non persisitent date attribute and use the formateDateTime command to create the string.
But Brian, I still think what you want can simply be done with the datetime attribute. Make sure that localize is off for this attribute that that when using a microflow on this attribute to change it always use the UTC versions of the command. Once you know the pitfalls it can be easiy....
Regards,
Ronald