Сделать шаг во времени по 30 мин

Я хочу сделать разделение времени по промежуткам. Чтобы высвечивалась не каждая минута, а только 00 или 30.

Как можно сделать так, чтобы время для приёма генерировалось от того времени, когда работает доктор

У меня есть модели для докторов и их графика работы doctor.models `

class DoctorUser(models.Model):
    user = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, primary_key=True)

    class Meta:
        verbose_name = 'Доктор'
        verbose_name_plural = 'Доктора'


    def __str__(self):
        """Returns the doctor's full name."""
        return f'{self.user}'


class DoctorProfile(models.Model):
    user = models.OneToOneField(DoctorUser, on_delete=models.CASCADE, primary_key=True)
    department = models.ForeignKey(Department, on_delete=models.PROTECT, null=True)
    experience = models.PositiveIntegerField(default=2)

    class Meta:
        verbose_name = 'Профиль доктора'
        verbose_name_plural = 'Профили докторов'

    def __str__(self):
        """Returns the doctor's full name."""
        return f"{self.user}"


WEEKDAYS = (
        ('Пн', 'Понедельник'),
        ('Вт', 'Вторник'),
        ('Ср', 'Среда'),
        ('Чт', 'Четверг'),
        ('Пт', 'Пятница'),
        ('Сб', 'Суббота'),
        ('Вс', 'Воскресенье'),
    )


class Schedule(models.Model):
    doctor = models.OneToOneField(DoctorProfile, on_delete=models.CASCADE)
    weekday = MultiSelectField(max_length=20, choices=WEEKDAYS)
    start_time = models.TimeField()
    end_time = models.TimeField()

    class Meta:
        verbose_name = 'График работы доктора'
        verbose_name_plural = 'Графики работы докторов'

    def __str__(self):
        days = ' '.join(self.weekday)
        return f"График доктора - {self.doctor} {days}"

**patient.models**

from django.db import models
from django.conf import settings
from django.contrib.auth.models import AbstractUser
from phonenumber_field.modelfields import PhoneNumberField
from doctor.models import DoctorProfile, Department, Schedule
import phonenumbers


class CustomUser(AbstractUser):
    is_patient = models.BooleanField(default=True)
    phone_number = PhoneNumberField(null=True, unique=True, region='KG')

    def __str__(self):
        return f'{self.first_name} {self.last_name}'


GENDER = (
    ('Male', "Мужчина"),
    ('Female', 'Женщина')
)


class UserProfile(models.Model):
    user = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='profile',
                                primary_key=True)
    gender = models.CharField(max_length=30, choices=GENDER)
    phone_number = PhoneNumberField(null=True, unique=True, region='KG')

    def __str__(self):
        """Returns the patient's full name."""
        return f"{self.user}'s profile"


class Appointment(models.Model):
    patient = models.ForeignKey(CustomUser, on_delete=models.CASCADE, null=False)
    referred_department = models.ForeignKey(Department, on_delete=models.CASCADE, null=False)
    referred_doctor = models.ForeignKey(DoctorProfile, on_delete=models.CASCADE, null=False)
    schedule = models.ForeignKey(Schedule, on_delete=models.CASCADE, null=False)
    appointment_date = models.DateField(null=False)
    appointment_time = models.TimeField(null=False)
    confirm_appointment = models.BooleanField(default=False, null=False)

    def __str__(self):
        return f"Приём у {self.patient}"

`