This has been asked before, and Pieter had a solution. Here’s a link to the previous question and his answer.
https://forum.mendix.com/link/questions/4399
You can validate through On change event microflow based on your requirment.
Create an on change microflow on the date / time attribute. You can then do the validation in the microflow and check for instance that the date is in the future or something like it.
Regards,
Ronald