How to add minutes to datetime in postgres via django

TL;DR: I want to add minutes to a datetime in postgres and can think of two ways to do it.

Consider the following django model:

from django.db import models


class AppointmentManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().annotate(
            end=models.ExpressionWrapper(
                models.F('start') + models.F('duration'),
                output_field=models.DateTimeField(),
            )
        )


class Appointment(models.Model):
    start = models.DateTimeField()
    duration = models.DurationField()

    objects = AppointmentManager()

Note that end is computed dynamically in the database by adding start and duration. This does exactly what I want.

However, I want the UI for the duration to be a number input that gives the duration in minutes. My current approach is to use the following form field:

import datetime
from django import forms


class MinutesField(forms.DurationField):
    widget = forms.NumberInput

    def prepare_value(self, value):
        if isinstance(value, datetime.timedelta):
            return int(value.total_seconds() / 60)
        return value

    def to_python(self, value):
        if value in self.empty_values:
            return None
        if isinstance(value, datetime.timedelta):
            return value
        if isinstance(value, str):
            value = int(value, 10)
        return datetime.timedelta(seconds=value * 60)

This looks a little brittle to me. Is it also possible to store the duration as integer and transform it to a duration dynamically? Something like this:

class AppointmentManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().annotate(
            end=models.ExpressionWrapper(
                models.F('start') + models.Func(models.F('duration'), function='minutes'),
                output_field=models.DateTimeField(),
            )
        )

class Appointment(models.Model):
    start = models.DateTimeField()
    duration = models.IntegerField()

    objects = AppointmentManager()