I've used

validate()

in the past. I have a page type where I have

'StartDate' => 'Date'

as a db field. I have the following validation method:

    /**
     * @return ValidationResult
     */
    public function validate()
    {
        $result = parent::validate();

        if ($this->ID > 0 && !$this->StartDate) {
            $result->error('Start Date is required');
        }

        return $result;
    }

I can create the page as

$this->ID > 0

works properly, but then after the initial creation, when

StartDate

is a valid date (using the date popup of the DateField) and I attempt to save the page, it throws the validation exception. Has anyone else come across this type of behavior?

Thanks

Did you try with $this-> StartDate(); ?

Btw if this is a required filed in the getCMSFIelds() function, why don't you use the native getCMSValidator() to set your fields as required?