admin panel, show date_created and date_updated? - django

Question

I tried following an answer at this previous post:
DateTimeField doesn't show in admin system
But maybe I'm just too dim to understand it.
No field of created_at shows up. Could anyone point me in the right direction?
model
class holding_transaction(models.Model):
holdingname = models.ForeignKey(holding, on_delete=models.CASCADE)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
admin.py
class holding_transactionAdmin(admin.ModelAdmin):
readonly_fields = ('created_at', )
admin.site.register(holding_transaction, holding_transactionAdmin)
Edit:

Answer 1

Update:
Here is the code that worked for me for an imaginary application called Beatles:
beatles/models.py:
from django.db import models
# Create your models here.
class Person(models.Model):
name = models.CharField(max_length=128)
created_at = models.DateTimeField(auto_now_add=True)
def __str__(self): # __unicode__ on Python 2
return self.name
beatles/admin.py
from django.contrib import admin
# Register your models here.
from beatles.models import Person
#admin.register(Person)
class PersonAdmin(admin.ModelAdmin):
readonly_fields = ('created_at', )
The answer to the question mentioned, states that this is not possible to happen.
Nonetheless, if you want to edit such fields, according to the docs you proceed as follows:
If you want to be able to modify this field, set the following instead
of auto_now_add=True:
For DateField: default=date.today - from datetime.date.today()
For DateTimeField: default=timezone.now - from django.utils.timezone.now()
If you want those fields just to be displayed, you can use the following code:
class YourModelAdmin(admin.ModelAdmin):
readonly_fields = ('created_at', 'updated_at', )
admin.site.register(YourModel, YourModelAdmin)