when the time of the date arrives, change the priority of the incident.

This is not possible out of the box.

Not exactly what you want, but you could use the following plugin as a basis to implement what you need https://github.com/mantisbt-plugins/Reminder/tree/Mantis-Version-2.X

There is a problem: operators do NOT have access to email, they can only see the mantis view monitor.

operators do NOT have access to email

Understand

But you could use the plugin as a basis to implement what you need.
I does a periodical check of a date you entered, and does something (send email) based on this date and the actual date.
Instead of sending email, you could change the plugin to set priority.
You even don't need to use a custom field for it, but can you use the due date standard field.
https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.config.duedate

Isn't this what you need?

Even without any plugin, this field is gets a red colored background if due date is reached.

duedate.PNG (7,845 bytes)   

duedate.PNG (7,845 bytes)   

Setting to resolved as there was no further feedback and the functionality can be implemented by writing a plugin.

That solution works for me, thank you very much for the attention.