Hi, i put this code inside a method but you can disable the method and try yourself to suit your needs :)

# this will generate pop up error message and date time module
from odoo.exceptions import UserError
from datetime import datetime

def disable_past_date(self):
    selected = self.env['your.Model'].browse(self._context.get('active_ids', [])) # This will get your current record form
    current_date = datetime.now() # this will get the today's date

    if self.selected_date and self.selected_date < current_date: # compare todays date and selected date
        raise UserError('The date selected is in the past.') # The error message

Best regards,

Altela (altelasoftware.com)