Confirm pop up on calendar

I have set up a form where if the user sets up a date in past the user will be prompted with a confirmation box.
However sometimes the confirmation dialog box does not block the next step. If I click next on calendar box the confirmation will trigger again and sometimes the if statement does not sometimes hold true if I pick today's date.

async validateDateRequired(args) {
        const value = args.value || "";
        if (value.trim().length === 0) {
            args.valid = false;
            args.message = "Date required";
        }

        if (DayPilot.Date.today() > args.value) {
           
        const modal =  await DayPilot.Modal.confirm("Date scheduled in the pass. Continue?");
            if (modal.canceled) {
                args.valid = false;
                args.message = "Valid date is required"
            }
        }
    },
////////////
assignmentForm(title, groups) {
        const form = [
            {name: title},
            {name: 'Group', id: 'group_id', type: 'select', options: groups, onValidate: this.validateGroupRequired },
            {name: "Start date", id: "start", type: "date", dateFormat: "d/M/yyyy", onValidate: this.validateDateRequired },
        ];

        return form;
    },
////////
const form = this.assignmentForm(title, responseJson.data);

        const modal = await DayPilot.Modal.form(form, item, { okText: 'Assign', cancelText: 'Close'}); 

        if (modal.canceled) {
            return;
        }

api call