serialize a datetime as an integer timestamp

REST_FRAMEWORK = {
    # if you want with milliseconds or
    'DATETIME_FORMAT': '%s.%f', 
    # only with seconds
    'DATETIME_FORMAT': '%s', 
}

Result in REST will be string

1. "1517863184.666435"

2. "1517863249"

If you want float(or integer) value in API, than you can use monkey patching.

Put the file monkey_patching.py in any of your apps and import it in app's __init__.py file. ie:

app/monkey_patching.py

#app/monkey_patching.py
import six
from rest_framework import ISO_8601
from rest_framework.fields import DateTimeField
from rest_framework.settings import api_settings


def to_representation_ext(self, value):
    if not value:
        return None

    output_format = getattr(self, 'format', api_settings.DATETIME_FORMAT)

    if output_format is None or isinstance(value, six.string_types):
        return value

    if output_format.lower() == ISO_8601:
        value = self.enforce_timezone(value)
        value = value.isoformat()
        if value.endswith('+00:00'):
            value = value[:-6] + 'Z'
        return value
    
    # FOR INTEGER RESULT 'DATETIME_FORMAT': '%s',
    # return int(value.strftime(output_format))

    # FOR FLOAT RESULT 'DATETIME_FORMAT': '%s.%f',
    return float(value.strftime(output_format))


DateTimeField.to_representation = to_representation_ext

app/init.py

#app/__init__.py
import app.monkey_patching

Tested with Django version 2.0.10 and Python 3.5.9

You'll want to write a custom serializer field, like so:

class TimestampField(serializers.Field):
    def to_native(self, value):
        epoch = datetime.datetime(1970,1,1)
        return int((value - epoch).total_seconds())

To support write operations you'd want to inherit from WritableField and also implement from_native().

EDIT for DRF 3.x & Python 3.8:

class TimestampField(serializers.Field):
    def to_representation(self, value):
        return value.timestamp()

If you want a JavaScript style timestamp:

class JsTimestampField(serializers.Field):
    def to_representation(self, value):
        return round(value.timestamp()*1000)