Datetime objects parsed as strings when signals used - Django 1.9.5

[Adrian Castellanos]

I found a strange behavior when accesing objects models using signals. The model is declared as:

class Campaign(models.Model):
    start_date = models.DateTimeField(null=True, blank=True)
    end_date = models.DateTimeField(null=True, blank=True)

When I use signals to connect the changes of the model with a function, a found that the instance that is passed as argument has strings instead of datetime in the dates:

The code of the conection between the model and the function is:

def export_campaign_to_mongo(sender, instance, *args, **kwargs):
    check_dates()
post_save.connect(export_campaign_to_mongo, Campaign)

This is the output of the console checking the types of the objects:

>>>type(instance)
<class 'campaigns.models.Campaign'>
>>>instance.start_date
'2018-04-11T00:00:00'
>>>from campaigns.models import Campaign
>>>Campaign.objects.first().start_date
datetime.datetime(2016, 10, 10, 0, 0)

I am using Django v1.9.5.