Custom schedule type is not being used

[sdrabblescripta]

Summary:

Custom schedule type is not being used, so is_due is never called.

• Celery Version: 5.3.5
• Celery-Beat Version: 2.5.0

Exact steps to reproduce the issue:

from app import is_last_day_of_month
CELERY_BEAT_SCHEDULE = {
        'foo': {
                'options': {'queue': DATABASE_NAME},
                'kwargs': {'creator_name': 'system'},
                'task': 'tasks.foo', # it doesn't matter what the task is, the issue is with the schedule class
                'schedule': is_last_day_of_month(run_every=120),
        },
    }

Last day of month func. Note that the `is_due` method is never executed - this is because the schedule class is always `schedule` and never `is_last_day_of_month`.

from celery import schedules

class is_last_day_of_month(schedules.schedule):

    def is_due(self, last_run_at: datetime) -> tuple[bool, datetime]:
        now = date.today()
        day_range = calendar.monthrange(now.year, now.month)
        eom = now.replace(day=day_range[1])
        return (now.day == eom.day, 86400)

Detailed information

Appears to be caused by this:

 class IntervalSchedule(models.Model):

   ...

    def schedule(self):
        return schedules.schedule(
            timedelta(**{self.period: self.every}),
            nowfun=lambda: make_aware(now())
        )